BACKGROUND:: Men who have sex with men (MSM) account for more than half of new HIV infections each year, and young Black MSM experience the highest incidence rates. Black MSM have not been found to engage in more HIV risk behaviors, and it has been proposed that sexual network factors (racially homophilous networks) and partnership characteristics (influence of older partners and familiarity with partners) may help account for this disparity. METHODS:: 143 ethnically diverse MSM were enrolled in an online prospective diary study of sexual behavior. Participants completed weekly diaries of sexual encounters and associated situational factors for 12 weeks. All analyses were conducted with Hierarchical Linear Modeling. RESULTS:: Black MSM reported significantly less unprotected sex than other groups and were the most racially homophilous group in terms of sexual partnerships. Having older sexual partners and familiarity with partners were both associated with increased odds of sexual risk in Black MSM only. A three-way interaction between participant age, participant race, and sexual partner age revealed a strong association between having older partners and odds of sexual risk for young Black MSM, as well as a strong association between having younger partners and sexual risk for older non-Black MSM. CONCLUSIONS:: Findings expand upon previous theory and cross-sectional research. Results indicate that some of the driving forces behind the disproportionate HIV incidence in Black MSM may be the greater likelihood of racially homophilous sexual networks combined with the stronger influence of sexual partner age and familiarity with partners on condom use.