Three families with late-onset 21 -hydroxylase deficiency were studied. Homozygous females presented with symptoms of mild hyperandrogenism such as acne, hirsutism, oligomenorrhea and menometrorrhagia. A homozygous male was asymptomatic and had reached normal adult height. The diagnosis of 21 -hydroxylase deficiency was based upon markedly elevated responses of plasma 17-hydroxyprogesterone during a short (30-min) ACTH infusion test. The propositi of two of the families were diagnosed despite long-standing glucocorticoid therapy and adrenal suppression by using a prolonged (48-hour) ACTH infusion. Heterozygotes of late-onset 21-hydroxylase deficiency had mildly elevated 17-hydroxyprogesterone responses to ACTH. Late-onset 21-hydroxylase deficiency was inherited as an autosomal recessive trait with close linkage to the histocompatibility leukocyte antigens. The B14 haplotype was present in all affected members. One affected female had a daughter with classic, salt-losing 21 -hydroxylase deficiency. Mixed heterozygosity of this patient for a classic and a late-onset 21 -hydroxylase deficiency allele may have caused the classic phenotype in her daughter (homozygote for 2 classic alleles).