The objective of this study was to determine how injection drug users' (IDUs) HIV-risk behavior differs with primary and other sex partners. Interviews were conducted with injection drug users from a needle exchange program (n = 243). Those with one sexual partner were more likely to report never using condoms with primary partners than were those with more than one partner (74% vs. 54%, p < 0.001). Those with more than one partner differed, between primary and other sexual partners, in their disclosure of HIV and IDU status, condom use, and drug use in combination with sex. Primary sexual partners of IDUs are placed at risk from IDUs' risk behavior with other sexual partners. Those planning HIV-risk reduction interventions for IDUs should consider risk behavior with primary partners separately from behavior with other partners. Evaluation of intervention effects should use partner-specific assessments of risk behavior.