To assess the influence of human immunodeficiency virus type 1 (HIV-1) infection on the natural history of acute hepatitis B virus (HBV) infection, a study was undertaken of the clinical records of all 77 homosexual men with documented seroconversion to anti-hepatitis B core antibody (anti-HBc) between visits to either of two Sydney clinics between 1985 and 1989. HIV-1-seropositive subjects developed chronic HBV infection (positive for hepatitis B surface antigen [HBsAg] greater than 6 months) more frequently (7/31, 23%) than HIV-1-seronegative ones (2/46, 4%; P = .026). HIV-positive subjects who cleared HBsAg had significantly more circulating CD4+ lymphocytes (mean, 547 x 10(6)/l) than those who did not (352 x 10(6)/l, P less than .005). A subset of subjects who acquired both viruses between visits had an even higher rate of chronic infection (4/10, 40%). Icteric illnesses were reported more frequently by HIV-1-seronegative (11/46, 24%) than -seropositive subjects (3/31, 10%; P = .20). These findings indicate a potential for an increased reservoir of HBV infection in the community as a consequence of the HIV-1 epidemic.