Risk factors for and serologic evidence of hepatitis B virus (HBV) infection were analyzed among 557 women. Study subjects were attending a clinic for sexually transmitted diseases and enrolled in a clinical trial of nonoxynol-9 prevention of gonococcal and chlamydial infections. Seventy-eight (14%) showed serologic evidence of past HBV infection. Only age at time of serum collection was significantly associated with HBV marker prevalence (P = 0.04). None of the four measures of sexual activity taken (number of sex partners per month, frequency of sexual intercourse, number of documented episodes of sexually transmitted diseases, and lifetime number of sexual partners) were significantly related to the presence of HBV markers. For each measure, however, differences observed between HBV positive and negative subjects were consistent with what would be expected if these factors did contribute to HBV infection risk. These results support the role of heterosexual transmission of HBV infection in women and are consistent with recommendations for hepatitis B immunization of heterosexually active persons with multiple sexual partners.