In an attempt to improve detection of heterozygote carriers of the gene for congenital adrenal hyperplasia (21-hydroxylase deficiency; CAH) 64 families with at least 1 affected member (72 homozygotes and 191 clinically healthy subjects) were studied by HLA genotyping and by the single-dose corticotropin stimulation test. Plasma samples were drawn immediately before corticotropin and 60 min after its injection, and they were analysed simultaneously for eight adrenal steroids by radioimmunoassay after extraction and automated gel chromatography. Heterozygosity was defined as the presence of one HLA haplotype in common with the affected relative. Of the various basal and corticotropin-stimulated steroid levels and their ratios, the ratio of 17-hydroxyprogesterone to 11-deoxycorticosterone after corticotropin had the greatest power to discriminate between heterozygotes and normal relatives; that ratio was significantly higher in the heterozygotes (n = 116) than in the normal relatives (n = 75) and there was no overlap between the groups (range 12.2-214 vs 1.2-11.9). Thus, it is possible to detect all CAH heterozygotes without examining the index case by means of specific steroid analysis.