The pattern of HLA-DR antigens was studied in a group of 66 patients with Addisonian pernicious anaemia, comprising a subgroup of 18 patients with associated endocrine disease and a subgroup of 48 patients with no associated endocrine disease. Compared with a control group of 120 subjects all 66 patients showed an increase in HLA-DR2 and DR4 and a decrease in DR3 (p less than 0.02). Significant differences were also found between the endocrine and non-endocrine subgroups for patterns of HLA-DR antigens (p less than 0.005) and for pairwise combinations of HLA-DR antigens (p less than 0.01). Relative to controls, the endocrine subgroup showed an increase of HLA-DR3/DR4 (relative risk 4.0), contrasting with an increase of HLA-DR2/DR4 (relative risk 6.85) and DR4/DR5 (relative risk 5.38) in the non-endocrine subgroup. These observations suggest that HLA-DR antigens or closely linked genes may interact to influence susceptibility to pernicious anaemia (or endocrine disease, or both). Thus interactive effects related to HLA-DR2/DR4 and DR4/DR5 may predispose to pernicious anaemia without endocrine disease, whereas interactive effects related to HLA-DR3/DR4 may predispose to pernicious anaemia in association with endocrine disease.