Mutations of the androgen receptor (AR) gene have been reported in prostate cancer, usually from tumor tissue specimens from late-stage, androgen-independent cancer. Occasionally, germ-line mutations have been found, but a link between AR mutations and predisposition to human prostate cancer has not been firmly established. Recently, two independent studies reported the same germ-line mutation at codon 726 in exon E (CGC to CTC) in two apparently unrelated Finnish prostate cancer patients. This arginine to leucine substitution was reported to alter the transactivational specificity of the AR protein. In the present study, the R726L mutation was analyzed by allele-specific oligohybridization in DNA specimens from 418 consecutive prostate cancer patients who reported a negative family history (sporadic group) and from 106 patients with a positive family history (hereditary group). The population frequency of the R726L mutation in blood donors was 3 of 900 (0.33%). In contrast, eight (1.91%) mutations (odds ratio = 5.8; P = 0.006) were found in the sporadic group, and two (1.89%) mutations were found in the hereditary group (odds ratio = 5.8; P = 0.09). Suggestive evidence of the segregation of the mutation with prostate cancer was seen in these two families. The present study indicates that the R726L substitution in the AR may confer an up to 6-fold increased risk of prostate cancer and may contribute to cancer development in up to 2% of Finnish prostate cancer patients. These results warrant additional large-scale studies of the significance of rare mutations and polymorphisms in candidate genes along the androgen signaling pathway as risk factors for prostate cancer.