Some recent epidemiologic studies have failed to confirm positive associations between insulin-like growth factor-I (IGF-I) and the risk of prostate cancer observed in earlier studies but have reported suggestive evidence for a positive association between IGF-binding protein-3 (IGFBP-3) and prostate cancer risk, a result contradicting the earlier assumption that high levels of IGFBP-3 would be protective against prostate cancer. We tested the association between IGF-I and IGFBP-3 and prostate cancer risk by measuring the two peptides in plasma samples collected at baseline in a prospective cohort study of 17,049 men. We used a case-cohort design, including 524 cases diagnosed during a mean of 8.7 years follow-up and a randomly sampled subcohort of 1,826 men. The association between each peptide level and prostate cancer risk was tested using Cox models adjusted for country of birth and alcohol consumption. The risk of prostate cancer was not associated with baseline levels of IGF-I or the molar ratio IGF-I/IGFBP-3 (all odds ratios are between 0.82 and 1.08; P(trend) > or = 0.2), whereas the risk increased with baseline levels of IGFBP-3 (P(trend) = 0.008), the hazard ratio (HR) associated with a doubling of the concentration of IGFBP-3 being 1.70 (95% confidence interval, 1.15-2.52). The HR for quartile 4 relative to quartile 1 of IGFBP-3 was 1.49 (95% confidence interval, 1.11-2.00). The HRs did not differ by tumor aggressiveness or age at onset (all Ps > or = 0.4). In our study, high levels of IGFBP-3 but not IGF-I were associated with an increased risk of prostate cancer.