In June 1982, sexual and other behavioral patterns were examined in 245 homosexual men in relationship to T-lymphocyte phenotypes that are characteristic of the acquired immunodeficiency syndrome (AIDS). Mean helper T-cell counts in New York City (579 +/- 32 cells/mm3) and Washington, DC, homosexual men with sexual contacts in areas at high risk (endemic) for AIDS (567 +/- 24 cells/mm3) were significantly lower than in Washington, DC, residents without such contacts (672 +/- 36 cells/mm3, p = 0.04 by analysis of variance). Helper T-cell counts in the Washington men were inversely correlated with a greater number of endemic-area homosexual contacts (p = 0.005), even after adjustment for multiple confounding variables (p = 0.02). The 31 Washington men with more than 15 endemic-area partners had a mean helper T-cell count of 517 +/- 44 cells/mm3, and 12 of those 31 men had helper T-cell counts less than 400 cells/mm3. AIDS patients are known to have a marked reduction in the number and function of helper T-lymphocytes. The data suggest that deficits of helper T lymphocytes can be acquired by homosexual contact with men in cities where AIDS is common. This supports the hypotheses that low helper T-cell counts may be caused by a sexually transmissible agent and that frequent homosexual exposure to residents of high-risk areas for AIDS may be an important means of spread of this agent.