This longitudinal study with male, predominantly Latino, injection drug users (IDUs) was undertaken to test a model of determinants of unprotected sex guided by the theory of planned behavior (TPB). Structural equation modeling (SEM) was used to test each theory-based predictor and the overall model's efficacy in predicting current and escalation of unprotected sex in a community sample (n = 371). The hypothesized model fit the data well—with intention and self-efficacy directly predicting risk behavior. In the observed model, the theory-based factors were significantly related to T1 and T2 unprotected sex outcomes, though the effects on the latter were mediated by T1 unprotected sex. The findings are generally congruent with postulates of the theory of planned behavior; however, greater predictive efficacy of future behavior may be achieved by considering psychosocial and other factors related to Latino male IDUs' sexual behaviors.