Two hundred and two homosexual men enrolled in a prospective cohort study of AIDS risk were assessed for differences in the occurrence and progression of human immunodeficiency virus type 1 (HIV-1) infection with respect to cigarette smoking. Among subjects who were initially seronegative, smokers were more likely than nonsmokers to become HIV-1 seropositive (p = 0.03). After seroconversion, serum beta 2-microglobulin and CD4+ lymphocyte levels were elevated in cigarette smokers relative to nonsmokers (p = 0.02 for both comparisons), but both of these differences disappeared within 2 years. There was no detectable difference in the risk of AIDS or Pneumocystis carinii pneumonia with respect to smoking. Our data suggest that cigarette smoking may alter the immune response to HIV-1 infection, but it appears to have no marked effect on clinical outcome. They also suggest that cigarette smoking may be a surrogate marker for continued high-risk sexual behavior in homosexual men.