Autoimmune destruction of the adrenal cortex is the most common cause of primary adrenocortical insufficiency (Addison's disease) in industrialized countries. We have investigated a large Norwegian cohort of patients with Addison's disease in terms of clinical manifestations, autoantibodies, and human leukocyte antigen (HLA) class II haplotypes. The study comprised 94 patients (54 females) of ages 6-85 yr (mean 45 yr) with, either isolated Addison's disease or part of autoimmune polyendocrine syndrome type II. Among those diagnosed before the age of thirty, 53% were men, while among those diagnosed at 30 or above, 30% were men. Altogether 77 (82%) of the 94 patients had autoantibodies against 21-hydroxylase (21OH). Thirty-eight of the 40 patients with disease duration 5 yr or less had such autoantibodies. This frequency fell to 60% among patients with a disease duration greater than 35 yr. Five women had gonadal failure. This failure correlated with antibodies against side-chain cleavage enzyme (P = 0.03). Antibodies against glutamic acid decarboxylase and IA2 correlated with the presence of type 1 diabetes (P < 0.005 and P = 0.003, respectively). The frequency of the HLA DRB1*03-DQA1*05-DQB1*02 (DR3-DQ2) and DRB1*04-DQA1*03-DQB1*0302 (DR4-DQ8) haplotypes were positively correlated to Addison's disease, whereas the DRB1*01-DQA1*0101-DQB1*0501 (DR1-DQ5) haplotype was negatively correlated. In addition, the DRB1*04 subtype DRB1*0404 was increased among Addison patients relative to controls. We verify that autoimmunity is the main cause of Addison's disease in our cohort. In younger patients, the disease is equally common in men and women. Measurement of autoantibodies against 21OH is a valuable tool in establishing the etiological diagnosis, especially in patients with a short disease duration. Addison's disease is associated with the DR3-DQ2 and DR4 (0404)-DQ8 haplotypes. A particularly high risk for disease development is observed when these occur in a heterozygous combination (DR3-DQ2/DR4-DQ8).