HIV seroprevalence was previously found to be 9-11% among the urban homeless in San Francisco, with most infections among injection drug users, men who have sex with men, and those who exchange sex for money or drugs. In this study, HIV incidence and risk factors for seroconversion were estimated among persons who repeatedly participated in cross-sectional serosurveys over a period of 10 years. There were 6 seroconversions among 799 HIV-negative persons, for an incidence rate of 0.30% per person-year (95% CI: 0.12-0.61% per person-year). The seroconversion rate among men who had had recent sex with men was 1.22% per person-year (95% CI: 0.29-3.18) and was 0.52% per person-year (95% CI: 0.19-1.13% per person-year) among those who ever injected drugs. The seroconversion rate among those who were aged <30 years at baseline was 1.67% per person-year (95% CI: 0.40-4.36% per person-year), which was significantly higher than for those age > or =30 years. Policy implications are that HIV resources aimed at the more stable, older homeless population should probably focus on treatment, while prevention efforts, conversely, should probably be targeted to younger homeless persons.