In countries not generally endemic for hepatitis B, homosexual men who are carriers of hepatitis B surface antigen (HBsAg) have to date largely been spared additional delta virus infection. A focus of homosexually acquired delta has, however, recently been identified in California. To assess the prevalence and risk factors for delta virus infection in homosexual men in Sydney, we tested the sera of 204 homosexual men with acute or chronic hepatitis B infection for total antibody to delta and delta antigen. Total antibody to delta was detected in eight men and delta antigen in one other (4.4% of the total). All men with antibody or antigen had intravenous drug use as a risk factor. Overall nine of 22 (40.9%) male homosexual intravenous drug users had serological evidence of delta infection. Delta positive patients tended to be younger, to have engaged in prostitution, to have more severe chronic liver disease, to be more likely to have a symptomatic acute illness, and to be less likely to express hepatitis Be antigen (HBeAg) in their sera. Though the homosexual transmission of delta infection cannot be concluded from this study, the detection of delta markers in nine homosexual men (including four prostitutes) suggests considerable potential for dissemination of this virus homosexually.