To investigate the enzymatic basis for abnormal steroid metabolism in subjects with male pseudohermaphroditism due to 5 alpha-reductase deficiency, the ring A reduced urinary 5 beta and 5 alpha metabolites of testosterone, androstenedione, 11 beta-hydroxyandrostenedione, cortisol and corticosterone were measured by gas chromatography. Assays of the four pairs of urinary 5 beta and 5 alpha steroid metabolites revealed decreased conversion of the parent steroids to 5 alpha-reduced urinary metabolites, with increased 5 beta to 5 alpha urinary steroid metabolite ratios. These studies establish that increased urinary 5 beta/5 alpha ratios are distinctive for this disorder, and represent the most reliable method for confirming the diagnosis of primary inherited 5 alpha-reductase deficiency. These data also suggest that the conversion on the many delta 4-3 ketosteroids to 5 alpha-reduced steroids may be due to a single enzyme with broad specificity, or multiple enzyme reductases with a common regulator.