Male pseudohermaphrodites (MPHs) with inherited 5 alpha-reductase deficiency and decreased dihydrotestosterone production have a global defect in 5 alpha-metabolism affecting both C19 androgen metabolism and C21 steroid metabolism. However, the decreased 5 alpha-reduction of testosterone to dihydrotestosterone is the only impaired steroid conversion to have clinical consequences, e.g., ambiguous genitalia, impaired prostate differentiation and development, and decreased facial and body hair. The 5 alpha-steroid metabolite profile in the MPHs was compared with that of men with benign prostatic hyperplasia who were administered varying doses of the 5 alpha-reductase inhibitor finasteride. Finasteride was found to be a potent inhibitor of both C19 androgen and C21 5 alpha-steroid metabolism affecting both hepatic and peripheral 5 alpha-metabolism. The 5 alpha-steroid metabolite profile was strikingly similar to that of MPHs with inherited 5 alpha-reductase deficiency. The data suggest that a 5 alpha-reductase gene codes for an enzyme with affinity for multiple steroid substrates.