This study examines an intervention for heterosexual couples to prevent human immunodeficiency virus/sexually transmitted infections. It also evaluates the effect of the intervention, which is based on current models of health behavior change, on intermediate outcomes (individual and relationship factors) and consistency of condom use. Eligible couples were administered a baseline interview and randomized to either a 3-session theory-based intervention or a 1-session standard of care comparison condition. Men and women completed 3-month interviews; only women completed 6-month interviews. No significant intervention effect on condom use was found among couples at 3 months (n = 212) or among women (n = 178) at 6 months. However, condom use increased significantly between baseline and 3 months and baseline and 6 months for participants in both treatment conditions. Intervention effects on condom use self-efficacy were found at 3 months and 6 months and on health-protective communication at 3 months. These findings provide valuable information for the design of future studies to help disentangle the effects of intervening with couples. (Contains 3 tables and 2 notes.)