Results from epidemiologic studies on the association between circulating carotenoid concentrations and the risk of prostate cancer are still inconsistent. We studied whether serum concentrations of carotenoids were associated with the risk of developing prostate cancer. The study population consisted of 997 middle-aged Finnish men (56.1 ± 6.6 yr) in the Kuopio Ischaemic Heart Disease Risk Factor (KIHD) cohort. Serum concentrations of carotenoids were measured by high-performance liquid chromatography. Subjects were classified into tertiles according to their serum concentrations of antioxidants. Relative risks (RRs) were estimated by using the Cox proportional hazard models. During the mean follow-up time of 15 yr, a total of 68 prostate cancer cases occurred. After adjusting for age, examination yr, family history of cancer, BMI, pack-yr of smoking, alcohol consumption, education, physical activity, serum total cholesterol, and serum a-linolenic acid, men in the highest tertile of serum concentrations of ß-carotene had 2.3-fold higher risk of prostate cancer as compared to those in the lowest tertile (RR = 2.29, 95% CI: 1.12-4.66; P = 0.023). a-Tocopherol and retinol were not associated with prostate cancer. This prospective study suggests that high-serum ß-carotene concentrations may increase the risk of prostate cancer in middle-aged men.