Controversy exists over the significance of associations between the SRD5A2 (5a-reductase type 2) polymorphisms, A49T and V89L, and risk of prostate cancer. These potentially functional polymorphisms may alter life-long exposure to androgens with subsequent effects on male health and aging. The aim of this study was to examine the association of these variants with prostate cancer risk, plasma hormone levels and androgenetic alopecia. Subjects include 827 cases and 736 controls from an Australian population-based case-control study of prostate cancer. Information on prostate cancer risk factors and patterns of balding were collected. Plasma levels of testosterone, 3a-diol glucuronide (3a-diolG), dehydroepiandrosterone sulfate, androstenedione, sex hormone-binding globulin and estradiol were measured for controls. No associations with the V89L polymorphism were found. Carriers of the rarer A49T A allele were at a 60% higher risk of prostate cancer (OR 5 1.60; 95% CI 1.09-2.36; p 5 0.02) and 50% lower risk of vertex and frontal balding (p 5 0.03) compared with men homozygous for the more common G allele. Although we found little evidence of association between this variant and plasma levels of 5 measured androgens, circulating 3a-diolG levels were 34% lower in A49T A allele carriers (p < 0.0001). Our study provides evidence that the SRD5A2 A49T A variant is associated with an increased risk of prostate cancer, lower levels of circulating 3a-diolG and decreased risk of baldness. These findings raise important questions with respect to previous assumptions concerning hormonal influences on prostate cancer risk in ageing males. ' 2006 Wiley-Liss, Inc.Key words: SRD5A2; polymorphisms; prostate cancer risk; population-based case-control study; plasma androgens; androgenetic alopecia Membrane-bound 5a-reductase type 2 (SRD5A2) is responsible for the irreversible conversion of testosterone into its more active metabolite, dihydrotestosterone (DHT) and is essential for the normal growth and development of the prostate gland.1 This hormone-regulator has been implicated in male pathophysiology, including prostate cancer and balding (androgenetic alopecia). The ratio of DHT to testosterone has been reported to be highest for African-Americans, intermediate for Europeans and lowest for Asian-Americans, which corresponds to reported ethnic-based risk of prostate cancer for these groups, suggesting an association with prostate cancer risk.2 Prostate cancer risk has been associated with vertex pattern balding.3 Increased levels of DHT have been demonstrated in the male balding scalp, 4 while lack of balding is seen in pseudohermaphrodites (men with congenital 5a reductase deficiency) with a concomitant reduced ability to convert testosterone to DHT.5 DHT in turn is metabolized to 5a-androstane-3a-17b-diol glucuronide (3a-diolG), a predictor of both cutaneous and intraprostatic serum 5a-reductase levels. 6 These observations support the ''androgen hypothesis'' that increases in active androgens (and their metabolites) are associ...