Researchers studying an abrupt slowdown in the rate of increase in AIDS cases believe medical therapy and fewer cases among homosexual and bisexual men may explain the phenomenon.

Whatever the reason, the decrease in the rate at which new AIDS infections are occurring has forced federal health officials to revise downward their estimates of cases expected to occur in the future.While the number of AIDS infections continues to increase, the graphs at the federal Centers for Disease Control show a slackening in the upward curve of cases.

In its latest projections of AIDS cases and prevalence estimates of the human immunodeficiency virus (HIV), the organism that causes AIDS, the CDC reported a 15 percent decline in projected cases for 1990, a 16 percent decline in estimated cases in 1991 and another 17 percent drop in case projections for 1992.

"The number of AIDS cases diagnosed per month continued to increase in 1987, but the rate of increase declined in the middle of that year, particularly among homosexual and bisexual men who did not use intravenous drugs," the report stated.

The CDC said the number of AIDS cases will continue to increase over the next four years.