One in three Americans is obese, a rate that has been steadily growing for more than two decades, but the health care system — in its attitudes, equipment and common practices — is ill prepared, and its practitioners are often unwilling, to treat the rising population of fat patients.

The difficulties range from scales and scanners, like M.R.I. machines that are not built big enough for very heavy people, to surgeons who categorically refuse to give knee or hip replacements to the obese, to drug doses that have not been calibrated for obese patients. The situation is particularly thorny for the more than 15 million Americans who have extreme obesity — a body mass index of 40 or higher — and face a wide range of health concerns.

Part of the problem, both patients and doctors say, is a reluctance to look beyond a fat person’s weight. Patty Nece, 58, of Alexandria, Va., went to an orthopedist because her hip was aching. She had lost nearly 70 pounds and, although she still had a way to go, was feeling good about herself. Until she saw the doctor.

“He came to the door of the exam room, and I started to tell him my symptoms,” Ms. Nece said. “He said: ‘Let me cut to the chase. You need to lose weight.’”