Human leukocyte antigen (HLA) alleles and plasma 17-hydroxyprogesterone levels after ACTH stimulation were studied in 134 German families of patients with the salt-wasting (SW), simple virilizing (SV), or nonclassical (NC) late-onset form of congenital adrenal hyperplasia (CAH). Unexpected hormonal evidence for CAH was found in 6 otherwise healthy members of the relatives' group, who, therefore, were considered to be NC cryptic cases. HLA typing revealed a genetic difference between the 2 classical disease forms; SW CAH was strongly associated with Bw47, whereas SV CAH was closely linked to B5(w51). It also confirmed the nearly complete connection of NC CAH with B14. These alleles, especially Bw47 and B14, are mostly components of normally rare haplotypes: A3,Bw47,DR7 and Aw33,B14,DR1, respectively. They do not occur in the families' disease-unaffected haplotypes. Thus, it may be that all or almost all individuals from the general population bearing 1 of them are in fact CAH heterozygotes. Moreover, it seems possible to predict the severity of an infant's disease from his genomic type. The HLA linkage data were consistent with those obtained from ACTH testing, which showed significantly higher 17-hydroxyprogesterone increases in the genetically defined heterozygous relatives of SW patients than in the respective members of SV families. Of the families, 2 were also informative for mapping of the CAH disease gene(s) within the HLA-B to Glo interval.