Insulin-like growth factor-I (IGF-I) plays an important role in prostate growth, hyperplasia, and carcinogenesis. Circulating IGF-I levels may be modulated by a genetic cytosine-adenine (CA) repeat polymorphism in the promoter region of IGF-I. The association of the polymorphism with the risk of prostate cancer and benign prostatic hyperplasia (BPH) was explored in 303 patients with prostate cancer, 219 patients with BPH and 262 controls. The number of CA repeats ranged from 15 to 22 in case and control subjects. The 19-CA-repeat allele (19-allele) was more frequently observed in both the prostate cancer and BPH patients compared with the controls (prostate cancer versus control: P<0.001; BPH versus control: P=0.001). Compared with non-carriers of the 19-allele, men homo-zygous for the 19-allele had a significantly increased risk of prostate cancer [age-adjusted odds ratio (aOR) = 3.36, 95% confidence intervals (CI) = 1.30-8.67, P=0.012] or BPH (aOR = 3.53, 95% CI = 1.32-9.46, P=0.012), and those heterozygous for the 19-allele also had an intermediate increased risk of prostate cancer (aOR = 1.78, 95% CI = 1.25-2.53, P=0.001) or BPH (aOR = 1.66, 95% CI = 1.14-2.43, P=0.009). A gene dosage effect for the aORs was found with an increasing number for the 19-allele (P<0.001 in prostate cancer and P=0.001 in BPH). No significant association was found between the presence of the 19-allele and the tumor stage and grade at the time of diagnosis. In conclusion, the 19-allele of IGF-I appears to increase the risk of prostate cancer and BPH with a gene dosage effect in the Japanese population.