The drug AZT increased the survival of AIDS patients overall by nearly five months, but too few patients get it, researchers said.

Doctors from Johns Hopkins University blamed the cost of AZT, some doctors' reluctance to treat AIDS patients and the small amount they said the government's Medicaid program pays doctors who treat indigent AIDS patients."I think our data show that the system has failed," said Dr. Richard Chaisson, the hospital's director of AIDS patient care.

The researchers studied 1,028 AIDS patients treated at Johns Hopkins from 1983 to 1989.

They reported in today's issue of the New England Journal of Medicine that from 1983 to 1985, before AZT was available, the patients lived an average of 310 days after diagnosis. Between 1987 and 1989, the average was up to 450 days.

Of the patients studied between 1983 and 1985, 15 percent were still alive two years after diagnosis, the study said. Of those diagnosed from 1987 to 1989, 35 percent were still alive after two years.

AZT was not available for AIDS treatment until 1987, and the researchers concluded that its introduction "made a sizable difference" in survival rates.

The researchers also concluded that young, white homosexual men were most likely to receive AZT. As a result, they said, survival rates for older patients, men who were infected with AIDS through heterosexual contact, women, minorities and intravenous drug users did not increase as much as those for white homosexual men.