Prospective investigations of circulating vitamin D concentrations suggest inverse associations with colorectal cancer risk, although inconsistencies remain and few studies have examined the impact of season. The authors conducted a prospective case-control study of 239 colon cancer cases and 192 rectal cancer cases (diagnosed in 1993-2005) and 428 controls matched on age and blood collection date within the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study, a cohort study of Finnish male smokers. Baseline serum 25-hydroxyvitamin D (25(OH)D) concentrations were categorized using a priori defined cutpoints of <25, 25-<37.5, 37.5-<50, 50-<75, and ≥75 nmol/L and by season-specific and season-standardized 25(OH)D quartiles. Conditional logistic regression models yielded multivariate-adjusted odds ratios for the predefined cutpoints of 0.63, 0.91, 0.73, 1.00 (referent), and 1.44 for colon cancer and 0.64, 0.58, 0.84, 1.00, and 0.76 for rectal cancer, respectively (all 95% confidence intervals included 1.00). Colon cancer risks were significantly elevated for the highest season-specific and season-standardized quartiles versus the lowest quartiles (OR = 2.11 (95% CI: 1.20, 3.69) and OR = 1.88 (95% CI: 1.07, 3.28), respectively), while rectal cancer risk estimates were null. These results provide no evidence to support an inverse association between vitamin D status and colon or rectal cancer risk; instead, they suggest a positive association for colon cancer.