Proanthocyanidins are polymers of monomeric unit flavan-3-ols with antioxidant, anti-inflammatory and free radical scavenging activities. We investigated the association between proanthocyanidin intake and prostate cancer risk through data that were collected between 1991 and 2002 in an Italian case-control study, including a total of 1,294 incident, histologically confirmed cases of prostate cancer and 1,451 controls admitted to hospital for acute, non-neoplastic, and non-hormone-related diseases. We estimated odds ratios (ORs) and their 95% confidence intervals (CIs) using multiple logistic regression models, and computed energy-adjusted proanthocyanidin intakes using the residual method. The ORs for the highest versus the lowest tertile were 0.80 (95% CI 0.83-1.00) for energy-adjusted monomers and dimers combined, 0.72 (95% CI 0.59-0.87) for polymers with ≥ 3 mers, and 0.72 (95% CI 0.59-0.88) for total proanthocyanidins. The inverse relation was stronger among cases with a Gleason score ≥ 7, with the ORs of 0.56 (95% CI 0.40-0.78) for monomers and dimers, 0.62 (95% CI 0.40-0.78) for polymers with ≥ 3 mers, and 0.57 (95% CI 0.42-0.77) for total proanthocyanidins. These risk estimates were consistent across strata of age, education, body mass index, and family history of prostate cancer. Our data indicate an inverse association between proanthocyanidins and prostate cancer risk.