In 43 cases of chronic active hepatitis of autoimmune-type, HLA-D locus-related antigens (HLA-DR) as well as A and B locus antigens were typed serologically. For 14 patients carrying HLA-B8 and DRw3 family members were typed to establish haplotypes. Control groups consisted of healthy persons and patients with chronic liver disease unrelated to autoimmune CAH (alcoholic cirrhosis and cryptogenic cirrhosis). In autoimmune CAH, DRw3 was significantly increased, 74%, compared with healthy controls, 32%, and liver controls, 31%, this increase being similar to the increase of HLA-B8. There was marked co-occurrence of HLA-B8 and DRw3 in 31 of 33 individuals positive for either antigen. DRw3 was present in 23 of the 24 cases with disease-onset before age 30 hr. HLA typing on the 14 families showed that a haplotype including B8 and DRw3 was present in all index patients; this haplotype also included A1 in 10 cases, A9 in 2 cases, and A2 and A28 in 1 case. The results indicate that in the autoimmune type of chronic active hepatitis occurring characteristically in young females, there exists a disease susceptibility gene strongly associated with the B and D locus of the major histocompatibility complex (MHC), and acting in concert with other non-MHC gene loci and/or environmental factors.