Purpose: Physical activity in colon cancer survivors has been associated with lower cancer recurrences and improved survival. Whether molecular features of the tumor portend more or less likelihood for benefit from exercise is unknown. Experimental Design: Using two large prospective cohort studies with physical activity assessments after colon cancer diagnosis, we examined expression of fatty acid synthase, p53, p21, and p27 and mutational status of K-ras and phosphatidylinositol 3-kinase(PI3KCA). We calculated hazard ratios (HR) of colon cancer-specific mortality, adjusted for tumor and patient characteristics, and tested for molecular interactions with exercise. Results: In a cohort of 484 men and women with stage I to III colon cancer, patients who engaged in at least 18 metabolic equivalent task (MET) - hours per week after diagnosis had an adjusted HR for colon cancer-specific mortality of 0.64 [95% confidence interval (95% CI), 0.33-1.23] and for overallmortality of 0.60 (95% CI, 0.41-0.86). A statistically significant interaction was detected based on p27 expression (P = 0.03). For tumors with loss of p27 (n = 195), physical activity of ≥18 MET-hours/week led to a HR for colon cancer mortality of 1.40 (95% CI, 0.41-4.72), compared with those with <18 MET-hours/week. However, for tumors with expression of p27 (n = 251), the adjusted HR was 0.33 (95% CI, 0.12-0.85). Molecular status of fatty acid synthase, K-ras, p53, p21, and PI3KCA did not influence the association between exercise and colon cancer-specific or overall mortality. Conclusion: The benefit of physical activity on outcomes in patients with stage I to III colon cancer may be influenced by p27 status. Further studies are warranted to confirm these findings.