Introduction: While previous studies indicate a significant relationship between self-efficacy and physical activity, less research has focused on this relationship among patients with type 2 diabetes. The purpose of this investigation was to examine whether self-efficacy mediated the relationship between participation in a 1-month, print-based physical activity intervention and improvements in activity levels. Methods: Participants (N = 85; mean age = 57; 73% Caucasian; 69% female) were recruited from a community diabetes center. The intervention was individually-tailored based on theoretical constructs, including self-efficacy. Results: After controlling for age, baseline activity, and baseline self-efficacy, the tailored intervention was associated with significant improvements in physical activity, 95% CI [23.01, 271.68] as well as self-efficacy, CI [0.02, 3.48]. There was an indirect effect of treatment on physical activity through self-efficacy, CI [0.77, 73.11], and the direct effect of treatment on physical activity was no longer significant, CI [-7.33, 253.40], after the influences of self-efficacy change were accounted for in the model. Discussion: Results supported a mediation effect, such that the treatment effect on physical activity was completely mediated by changes in self-efficacy. Although replication is needed, results support the theoretical rationale for targeting self-efficacy to promote physical activity among patients with type 2 diabetes. © 2009 Springer Science+Business Media, LLC.