BACKGROUND: Studying the heterogeneity and correlates of HIV risk in the sexual networks of black and white men who have sex with men (MSM) may help explain racial disparities in HIV-infection. METHODS: Black and white MSM were recruited as seeds using venue-based time sampling and provided data regarding their recent sex partners. We used chain referral methods to enroll seeds' recent sex partners; newly enrolled partners in turn provided data on their recent sex partners, some of whom later enrolled. Data about unenrolled recent sex partners obtained from seeds and enrolled participants were also analyzed. We estimated the prevalence of HIV in sexual networks of MSM and assessed differential patterns of network HIV risk by the race of the seed. RESULTS: The mean network prevalence of HIV in sexual networks of black MSM (n = 117) was 36% compared with 4% in networks of white MSM (n = 78; P < 0.0001). Sexual networks of unemployed black MSM had a higher prevalence of HIV than their employed counterparts (51% vs. 29%, P = 0.007). The networks of HIV-negative black MSM seeds aged 18 to 24 years had a network prevalence of 9% compared with 2% among those aged 30 years or older. In networks originating from a black HIV-positive seed, the prevalence ranged from 63% among those aged 18 to 24 years to 80% among those 30 years or older. CONCLUSIONS: The high prevalence of HIV in the networks of HIV-negative young black MSM demonstrates a mechanism for the increased HIV incidence observed in this age group. More research is needed into how age and socioeconomic factors shape sexual networks and HIV risk.