We found that among men with higher serum IGF-2 concentrations, the association of 25(OH)D with prostate cancer was modified by treatment arm (OR, finasteride: 0.84, 95% CI: 0.62–1.14; OR, placebo: 1.33, 95% CI: 1.00–1.65); p for interaction = 0.04). The gene discussed is IGF2; the disease is prostate cancer.