In fact, at San Francisco General, intensive care doctors have decided to divert ambulances to other hospitals rather than treat critically ill patients in the emergency room.

New York City hospitals, burdened by overcrowding, are increasingly closing their emergency rooms to patients and diverting ambulances to less crowded hospitals, city hospital and ambulance officials said yesterday.

Twenty-two of the 37 private hospitals that are part of the city's 911 emergency system were diverting ambulances from their overcrowded emergency rooms, the report said.

In a 1999-2000 study of ambulance diversions and death from heart attacks in New York City, my colleagues and I found that, on average, three hospitals a day diverted ambulances to other hospitals for periods of approximately five hours each.

The hospital asked all visitors to leave and diverted ambulances for an hour and a half, beginning at about 2 p.m., said Jim Mandler, a hospital spokesman.

Officials diverted ambulances to other hospitals while they quarantined the woman and two friends who had accompanied her in a section of the emergency room, said Bill Green, the hospital's vice president of external affairs.

When its emergency room got too crowded over the weekend, for instance, Ms. Dale said the hospital was forced to divert ambulances to other hospitals.

"They are diverting ambulances to another hospital because their staff is full to capacity," Mr. Doheny said.

The torrid weather sent so many people to emergency rooms that at one time or another, nearly 20 hospitals had to go on a treatment-delaying status in which they divert ambulances and patients to other centers.

Last year, many hospitals were near capacity and occasionally had to divert ambulances to other hospitals.