We compared sexual risk behaviors of men who have sex with men and inject drugs (MSM/IDU) with those of other men who have sex with men (MSM). Of 910 MSM surveyed, 106 (12%) injected drugs in the previous year. MSM/IDU were younger than MSM and more likely to be HIV-seropositive, Aboriginal, economically disadvantaged, engaged in the trade of sex for money or drugs, and to report having female sexual partners. MSM/IDU reported more casual sexual partners and in multivariate analyses were twice as likely to report unprotected receptive anal intercourse with casual partners. These results, combined with those from previous analyses, suggest that the higher risk for HIV seroconversion among MSM/IDU in this cohort is attributable mainly to sexual rather than injection-related exposures. Controlled assessments are needed to identify optimal sexual risk reduction strategies for MSM/IDU.