This study investigated whether gender-role related traits agency and communion contribute to successful health behavior change, in an interplay with domain-specific psychosocial factors, namely, agency, mediated by health-related self-efficacy, and communion, moderated by social support. Data from women (N = 282) participating in the GOAL Lifestyle Implementation Trial were analyzed using structural equation modeling. Agency and increase in self-efficacy both independently predicted waist circumference reduction in the 1-year follow-up. Individuals high in communion succeeded in waist reduction only if they received social support. Initial self-efficacy increase predicted 3-year waist reduction. Gender-role orientation, together with social environment, influences behavior change intervention outcomes.