he patient, a 38-year-old man at a small hospital in Providence, R.I., was suffering intense pain from a bowel obstruction. But when his doctor recommended an operation, the man refused, saying he was afraid of surgery.

Dr. Joan Teno, a member of the hospital's ethics committee, said that even though the man was bright and articulate, with no history of mental illness, his internist thought he did not understand the risk of refusing surgery.

A psychiatrist determined that the patient's mental capacity was impaired. A court declared him medically incompetent and made his mother his guardian. She consented to the surgery, which was performed successfully. The man recovered and went back to work and, Dr. Teno said, expressed thanks that someone had intervened.

Some experts say many seriously ill people who appear competent may actually have impaired abilities to make complicated medical decisions. Often, experts say, this impairment goes unrecognized, and regrettable decisions are made.

Patients may refuse surgery or chemotherapy, only to have their diseases become life-threatening; they may consent to join experimental studies without fully understanding the risks; they may make life-and-death decisions like signing do-not-resuscitate orders.

But other experts disagree, saying that even the most seriously ill patients have the capacity to make decisions, and that to think otherwise will throw health care back 20 or 30 years to a time when doctors paternalistically told patients what to do.

"There's been a long-term concern by physicians that patients, either because of injury, disease, anxiety or too much pain, don't know what they're doing," said Dr. Richard Zaner, a professor of medical ethics and philosophy of medicine at Vanderbilt University Medical Center in Nashville. "As far as I know that's pretty much unfounded."

Few studies have examined the mental capacity of sick patients. In June, Dr. Eric J. Cassell, professor of public health at Weill Medical College of Cornell University, published a small study in The Annals of Internal Medicine, measuring the decision-making abilities of 63 patients who had serious illnesses like AIDS, pneumonia or cancer, or had undergone major surgery like a coronary artery bypass. The patients had proved mentally competent on a test called the Mini Mental Status Examination.

They were given seven relatively simple tasks that measure judgment. After completing the tasks, 15 of 24 very sick people had no correct answers, for a median score of zero, compared with the controls, who had a median score of six.

Dr. Cassell concluded that the decision-making abilities of the very sick were similar to those of children under 10. "We showed that people could be mentally capacitated but may not have the capacity to make these types of decisions," he said, adding that more studies were needed.

In another study, published in the journal Psychosomatics in 1997, 82 heart attack patients performed as well as healthy people on decision-making tests. "They looked to be functioning like the person on the street," said the study's author, Dr. Paul Appelbaum, chairman of the psychiatry at the University of Massachusetts Medical School. But, he said, 10 percent of the patients had substantial difficulty acknowledging the nature and severity of their disease, a problem that could call their decision-making capacity into question.

Two other studies found that physicians underestimated their patients' degree of cognitive impairment.

Other experts are worried about reversing trends toward patient autonomy. "The thing I object to is beginning with the presumption that people can't make decisions with severe pain or anxiety," said Dr. Zaner of Vanderbilt. "You don't know that. I've known some patients in pretty severe pain who knew exactly what they wanted and were very clear about it."

Typically, doctors hesitate to question a patient's competence. "This is the struggle every clinician deals with on a daily basis," said Dr. Teno, a geriatrician and expert in end of life care at Brown Medical School. "How do you make decisions with patients of varied cognitive capacity? The Cassell study brings up the possibility that we may need to be raising questions about people's capacity more often."

Decision making requires the ability to understand complex medical information and its relevance to one's own situation, to reason with the information, to see things from different perspectives and to say yes or no in a consistent way.

Those who may be at greatest risk for impaired decision making include patients whose condition or treatment is likely to cause dementia, like those with obstructive pulmonary disease, AIDS, brain cancer, head injuries, depression or schizophrenia. People receiving high doses of steroids, opiates or sedatives are also at risk, as are the elderly, who run a high risk of changes in their mental status when hospitalized.

"Within these groups, there are more likely to be people that look intact to the casual observer, but if one looked more closely, are having a fair amount of trouble making their decisions," Dr. Appelbaum said. These groups may make up a larger proportion of the patient population than physicians have realized, he said.

"In our experience, we are more likely to see errors made in the direction of assuming competence on the part of people who may be fairly impaired than incorrectly assuming incompetence in people who are not impaired," Dr. Appelbaum said.

These errors may be the result of the pendulum's having swung too far toward autonomy. In discussing treatments, many doctors now use a model called shared decision making, in which the doctor explains the options and possible outcomes, the patient discusses concerns, values and preferences, and together they reach a decision that's best for each patient. What happens more often is that doctors offer the options, but leave the burden of deciding to the patient. That may work for many patients, Dr. Cassell said, but not for the seriously ill.

"I think it's grossly unfair and I actually think it's an abuse of a patient to put someone in a position to make decisions when they don't have the capacity to make them," he said.

"What these people really need is somebody who is able to find out what is in their best interest, someone to help them come to a decision that represents their beliefs." But, Dr. Cassell added, few doctors have the skills to do that.

In hospitals, where patients are treated by many doctors, they may find themselves discussing important decisions with someone who doesn't know them. These doctors may be less aware of what is normal for a patient and what is out of character, and therefore less likely to pick up impaired thinking. Often, doctors do not have time to stay with a patient long enough to detect the type of subtle mental impairment that affects decision making. And most physicians have not been trained to think about impaired decision making in the first place.

When physicians do sense a problem they can seek help from the hospital's ethicist or ethics team — though not all hospitals have one. The family may also be asked to help. However, research from the large-scale Study to Understand Prognoses and Preferences for Outcomes and Risks of Treatment, conducted from 1989 to 1997, found that family members and doctors were often unaware of patient preferences regarding do-not-resuscitate orders or other life-and-death decisions.

"In many cases family err on the side of doing more than the patient wanted, either because they don't know or because they don't want to feel guilty later on that they didn't do enough," said Dr. Joel Tsevat, president-elect of the Society for Medical Decision Making in Washington.