HLA markers (A, B, C, DR loci) were determined for the members of 52 unrelated families with at least one child suffering from congenital adrenal hyperplasia due to 21 hydroxylase deficiency, permitting genotyping. The gene frequencies of the 52 index cases were compared with those obtained from the patients' normal haplotypes and with those of a control reference panel. No significant differences were observed, except a clear decrease in the frequency of HLA-B8 among the haplotypes that carry the gene for congenital adrenal hyperplasia.