To investigate the genetic polymorphisms of the HLA region and the molecular defect of the P450c21B gene in congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency, we studied 89 individuals from 25 families of CAH patients (14 classical forms, 11 non-classical forms). The following immunogenetic and hormonal investigations were performed: HLA-A and B typing, restriction fragment length polymorphism (RFLP) analysis of 21-hydroxylase A and B genes, and serum 17-OH-progesterone values determined basally and 60 min after ACTH stimulation. In the patients affected by the classical form, RFLP analysis revealed 5 deletions and 1 gene conversion in 6 haplotypes and no molecular defect in the others, who probably carry point mutations. In the patients with non-classical form we found P450c21A duplication in 11/18 haplotypes; 9 of the 11 patients shared the HLA-B14 allele. Utilizing both hormonal and genetic data we identified two cryptic forms; hormonal data alone failed to differentiate heterozygous from normal individuals.