A cancer diagnosis can trigger change in both lifestyle behaviors and mental health outcomes such as ‘growth’ and ‘benefit-finding’. Assuming changes in mental health outcomes are based upon changes in specific behaviors, the Theory of Planned Behavior (TPB) may facilitate understanding of post-diagnosis change in physical and psychosocial ‘health’ behaviors. Adults (n=130) ⩽2 years post-cancer diagnosis completed an internet survey. Current performance and future behavior intentions for two physical (e.g. eating a healthy diet) and four psychosocial (e.g. spending quality time with family/friends; engaging in spiritual or religious activities) health behaviors were assessed. TPB constructs (subjective norm, behavior attitudes, perceived behavioral control) for each of the six behaviors were also assessed. Multiple regression analyses indicated the set of TPB constructs accounted for an increment of 25–53% of variance in behavioral intentions beyond that accounted for by clinical and demographic variables. Among individual TPB constructs, behavioral attitude was most consistently associated with behavioral intentions while subjective norm was least consistently associated with behavioral intentions. The TPB could serve as a comprehensive model for understanding change in both physical and psychosocial health behaviors after cancer diagnosis and could suggest innovative approaches to developing interventions to enhance post-diagnosis ‘growth’ and ‘benefit finding’. Copyright © 2005 John Wiley & Sons, Ltd.