Homosexual men afflicted with a mysterious, and often deadly, new medical syndrome have been found to suffer from a severe breakdown in their natural defense mechanisms, three medical teams report.

A series of studies by a total of 32 researchers in New York and California in this week's New England Journal of Medicine provides the first clinical documentation that an unexpected outbreak of rare infections and cancer is found in homosexuals whose immune systems make them highly susceptible to organisms normally harmless in healthy individuals.

The federal Centers for Disease Control sounded an alert last summer that surprising cases of a rare form of pneumonia and of skin cancer were cropping up, sometimes in combination, in young homosexuals who would not seem likely to get these illnesses.

The latest count, as of Dec. 4, totals 180 individuals--largely homosexual males--who have been found to have a cancer known as Kaposi's sarcoma, a pneumocystis pneumonia or other infections not thought to be the result of previous diseases.

Overall, more than 40 percent of the cases reported to the CDC and two-thirds of the 19 patients described in the New England Journal have died of what appears to be an entirely new syndrome involving more than a dozen types of organisms.

More than 80 percent of the cases come from New York City, Los Angeles and San Francisco, but one death in a young District of Columbia homosexual was reported. The cases continue to be reported at the rate of five or six each week.

The cancer and pneumonia have previously been known to occur more frequently in patients with damaged immune systems, the result of drug treatment--cancer drugs or drugs administered to those undergoing kidney transplants--or underlying disease. But scientists are puzzled as to why these ailments should appear in an even more virulent form in young homosexuals.

Most of the 19 described in the New England Journal had pneumocystis pneumonia as well as other infections, three had the cancer as well and four had severe skin ulcers. Prior to getting the life-threatening diseases, many were reported to have suffered for several months from undiagnosed illness, including fatigue, fever and weight loss. In all, the infection-fighting white blood cell counts fell far below normal.

David T. Durack, chief of infectious diseases at Duke University Medical Center, notes that the new studies provide the "first data showing that the defect is primarily a cellular immune deficiency. That means that the lymphocytes, one of the important white blood cells, are not functioning normally and infections take the opportunity and cause serious disease."

He says there are several theories as to why homosexual men are affected, including increased exposure to viral diseases and drugs that might affect the immune system, genetic predisposition or combinations of the three.

The case reports suggest that many of the victims showed evidence of past or current infection with cytomegalovirus, a form of herpes virus known to suppress the immune system in animals and humans and commonly found in high levels in semen.

However, since this form of virus has been around for a long time, Durack, in a New England Journal editorial, notes that "some new factor"--possibly recreational drugs--may also play a role. One suspect is the nitrite group of drugs that are commonly inhaled to enhance sexual response.