HLA (human leucocyte antigens) alleles and plasma 17-hydroxyprogesterone levels after ACTH stimulation were studied in 134 German families of patients with salt-wasting (SW), simple virilizing (SV), and nonclassical (NC) late-onset forms of CAH. HLA typing revealed a genetic difference between the two classical disease forms. SW-CAH was strongly associated with Bw47 and SV-CAH was closely linked to B5. The nearly complete connection of NC-CAH with B14 was confirmed. Bw47 and B14 were mostly components of the normally rare haplotypes A3, Bw47, DR7 and Aw33, B14, DR1, respectively. They did not occur in the families' disease-unaffected haplotypes. The HLA linkage data were consistent with those obtained from the ACTH stimulation test which showed a higher 17-hydroxyprogesterone increase in the group of genetically defined heterozygous relatives of SW patients than in the groups of heterozygous members of SV and NC families.