Despite considerable research, the causal relationship remains unclear between HIV/AIDS complacency, measured as reduced HIV/AIDS concern because of highly active antiretroviral therapy (HAART), and HIV risk behavior. Understanding the directionality and underpinnings of this relationship is critical for programs that target HIV/AIDS complacency as a means to reduce HIV incidence among men who have sex with men (MSM). This report uses structural equation modeling to evaluate a theory-based, HIV/AIDS complacency model on 1,593 MSM who participated in a venue-based, cross-sectional survey in six U.S. cities, 1998-2000. Demonstrating adequate fit and stability across geographic samples, the model explained 15.0% of the variance in HIV-acquisition behavior among young MSM. Analyses that evaluated alternative models and models stratified by perceived risk for HIV infection suggest that HIV/AIDS complacency increases acquisition behavior by mediating the effects of two underlying HAART-efficacy beliefs. New research is needed to assess model effects on current acquisition risk behavior, and thus help inform prevention programs designed to reduce HIV/AIDS complacency and HIV incidence among young MSM.