Association of hepatitis B surface antigen and core antibody with acquisition and manifestations of human immunodeficiency virus type 1 (HIV-1) infection. We examined the associations between seropositivity for hepatitis B virus (HBV) with the presence or development of antibodies to human immunodeficiency virus (HIV-1) and with HIV-1 induced T-helper lymphocyte deficiency or acquired immunodeficiency syndrome (AIDS). Serologic data on HBV and HIV-1, cytometric enumeration of CD4+ lymphocytes, clinical events (AIDS by Centers for Disease Control criteria) and hepatitis B vaccination histories were available on 4,498 homosexual participants in the Multicenter AIDS Cohort Study, Men were classified as to previous infection with HBV and prevalent or incident infection with HIV-1. Although there was an association between seropositivity for HBV infection and HIV-1 infection at enrollment (odds ratios anti-HBc 2.6; HBsAg 4.2), the relation between HBV seropositivity and subsequent seroconversion to HIV-1 was weaker (odds ratios 1.3 and 1.6). HIV-1 seroconversion was also associated with a history of certain other sexually transmitted diseases, but predisposing sexual practices did not account for the association between HBV and HIV-1 infection. Seropositivity for HBV infection at entry was not related to initially low or more rapid subsequent decline in T-helper lymphocyte counts and was not associated with an increased incidence of AIDS during 2.5 years of follow-up. History of vaccination against HBV did not appear to decrease susceptibility to HIV-1 infection or to subsequent progression of immunodeficiency. We conclude that prior HBV infection is unlikely to be specifically associated with acquisition of HIV-1 infection and is unrelated to more rapid progression of HIV-1-induced immunodeficiency. 