To examine the extent of hepatitis B virus infection (HBV) in an inner-city community, we determined the prevalence, incidence, and correlates of HBV seroreactivity in a representative sample of unmarried whites, African-Americans, and Hispanics living in San Francisco during 1988 to 1989 and again 1 year later in 1989 to 1990. Unmarried men and women aged 20 to 44 years were surveyed in a random household sample drawn from three neighborhoods of varying geographic and cultural characteristics. Hepatitis B infection was determined by testing specimens for antibody to hepatitis B core antigen (anti-HBc). Of blood samples available from 1,108 participants from the initial survery, 159 (14%) were anti-HBc positive. There was a strong positive association between anti-HBc positivity and positive serological tests for human immunodeficiency virus, herpes virus type 2, and syphilis. In women and heterosexual men, after controlling for other variables, anti-HBc positivity was significantly associated with older age (P < .001), nonwhite ethnicity (P < .01), less education (P < .05), injection drug use (P < .001), being paid for sex (P < .05), and lifetime number of sexual partners (P < .05). Among homosexually active men, after controlling for other variables, anti-HBc positivity was significantly associated with nonwhite ethnicity (P < .001), injection drug use in a sexual partner (P < .05), and number of lifetime sexual partners (P < .05). There were 19 (3.2%) incident HBV infections. Participants who used injection drugs (relative risk [RR], 8.2; 95% confidence interval [CI] 3.9 to 17.4), crack cocaine (RR, 3.5; 95% CI, 1.2 to 9.6), or who were paid for sex (RR, 7.6; 95% CI, 1.4 to 41.1) were more likely to have recently acquired HBV than participants who did not practice these activities. HBV antibodies were found in 14% of an ethnically diverse community based sample, and in general were associated with serological evidence of and risk behaviors for sexually transmitted diseases as well as with injection drug use. Subjects with incident compared with those with prevalent HBV infections were younger, more commonly heterosexual, and more commonly illicit drug users. (HEPATOLOGY 1995 22:44–49).