In Ontario, persons with multiple sexual partners became eligible for free hepatitis B vaccine in late 1991. From January 1992 to January 1993, a sexually transmitted disease (STD) clinic for men in Toronto was one site where the vaccine was delivered. The objectives of the study were to determine the seroprevalence of hepatitis B markers, the series completion rate for hepatitis B vaccine, and the seroprevalence of hepatitis B markers required before screening would be a cost-effective endeavour. The seroprevalence of anti-HBs in gay/bisexual men was 18.1% (95% CI: 15.3%-20.7%) and in heterosexual men was 5.1% (95% CI: 3.0-9.7%). HBsAg was detected in 0.9% (95% CI: 0.04%-1.8%) of gay/bisexual men and was undetected in heterosexual men. Completion of a full vaccine series was 47% for gay/bisexual men and 25% for heterosexual men (p = 0.04). A combined seroprevalence of HBsAg and anti-HBs of at least 30-64%, depending on cost figures used, was required to justify screening prior to completion of a full series.