This nonblinded randomized controlled trial investigated the efficacy of a physical activity (PA) intervention underpinned by self-determination theory. Participants (N = 31, mean age 69 years [SD = 4.9]) diagnosed with bowel polyps were randomized to an active lifestyle program (ALP; n = 17) or standard care (n = 14). ALP received supervised exercise and counseling for 6 months. Both groups were followed up at 12 months. Outcomes were change in PA and behavioral regulation. Data were analyzed with intention to treat. At 6 months, differences were observed for behavioral regulation in favor of ALP (p < .05). PA differences were significant for leisure, walking, and vigorous in favor of ALP (p < .05). The self-determination theory can be an effective strategy for promoting PA behavior change in this population, but a larger trial is needed to further explore the utility of the self-determination theory in this context.