The plasma concentrations of mineralocorticoid hormones, basal and after stimulation and suppression with ACTH, can identify the heterozygotes in a family with two siblings with 17 alpha-hydroxylase deficiency. Both parents and one sibling had elevated levels of plasma deoxycorticosterone, corticosterone, 18-hydroxydeoxycorticosterone, and 18-hydroxycorticosterone, but normal cortisol and aldosterone concentrations. Stimulation with ACTH effected additional increases in the elevated steroid and cortisol levels, but not in aldosterone, further increasing the discrepancy and the ratio between 18-hydroxycorticosterone and aldosterone. One sibling had normal steroid patterns and an 18-hydroxycorticosterone to aldosterone ratio. Suppression of ACTH restored the steroids to low normal levels. In addition, the ratio of the gas chromatographic analysis of the total major urinary metabolites of corticosterone to total metabolites of cortisol was greater, and the sum of urinary androsterone and etiocholanolone to total corticosterone and cortisol metabolites was less in the heterozygotes than in normal subjects. This identifies deficient 17-hydroxylation, which is required for the production of cortisol and C-19 steroids. These criteria appear unique for the 17 alpha-hydroxylase defect in the heterozygote.