Objective: This study examined change in self-efficacy as a mediator of the effects of a mailed print intervention on the dietary and exercise practices of newly diagnosed breast and prostate cancer survivors. Method: A total of 543 breast and prostate cancer patients were recruited from 39 states and two provinces within North America. Participants were randomly assigned to receive a 10-month program of tailored mailed print materials that aimed to increase fruit and vegetable consumption, reduce fat intake, and/or increase exercise or a 10-month program of publically available materials on diet and exercise. Telephone surveys conducted at baseline and 1 year assessed dietary practices, physical activity, and self-efficacy for engaging in these health behaviors. Results: Results indicated that changes in self-efficacy for fat restriction and eating more fruits and vegetables were significant mediators of the intervention's effects on dietary outcomes at 1-year follow-up. The intervention did not significantly affect self-efficacy for exercise; however, a significant, positive relationship was found between self-efficacy for exercise and exercise duration at follow-up. Conclusions: Findings are largely consistent with Social Cognitive Theory and support the use of strategies to increase self-efficacy in health promotion interventions for cancer survivors. Copyright © 2008 John Wiley & Sons, Ltd.