The association of HLA antigens and type I or "lupoid" CAH-C was investigated in a population of 52 Argentinian Caucasoid patients. When compared with a population of normal individuals of the same ethnic group (n = 197), a significant increase of HLA-DR6 was observed (68.6% in patients vs 17.3% in controls; RR = 12.3, chi 2 = 52.4, pc = 0.00001). DNA typing showed that the HLA-DRB1*1301 allele was present in 32 out of 33 HLA-DR6 patients (66.6% of all the C-CAH patients vs 10.5% in controls; RR = 16.2, chi 2 = 111.3, pc = 0.00001). Analysis of HLA-DQB1 alleles also showed a significant increase of DQB1*0603 (RR = 15.4, chi 2 = 106.5, pc = 0.00001), an allele found in strong linkage disequilibrium with DRB1*1301. The association of CAH-C with this particular HLA-DR6 haplotype has not been previously described for the adult onset CAH. This different HLA predisposition, together with the fact that extrahepatic autoimmune diseases occur frequently only in the adult form of the disease, suggest that the immunopathogenic mechanisms involved in the development of these diseases may be different.