The PRECEDE model for health promotion proposes three types of influences on health behaviors: Predisposing, Enabling, and Reinforcing factors. This model was used to examine a range of influences on HIV risk behaviors (sharing syringes and other injection-related paraphernalia) among Puerto Rican injection drug users (IDUs). A total of 698 IDUs were interviewed (438 in East Harlem, New York, and 260 in Bayamón, Puerto Rico). Both types of risk behaviors were more prevalent in Puerto Rico. Similarities in influences on syringe sharing behaviors were found in the two sites and included self-efficacy (for reducing injection-related sharing) and norms. Influences on the sharing of other injection-related paraphernalia were primarily Enabling factors in both communities, and purchasing drugs with others was the strongest predictor of paraphernalia sharing. The need to address risks associated with joint drug purchasing in both locations and to enhance efforts to reduce risks among IDUs in Puerto Rico is indicated.