Patients with autoimmune polyendocrine syndrome type I (APS I) have autoantibodies against the enzyme aromatic L-amino acid decarboxylase (AADC) of pancreatic beta-cells. The aim of the present study was to investigate the presence of anti-AADC antibodies in a large cohort of patients with APS I, and in patients with isolated insulin-dependent diabetes mellitus (IDDM). We found autoantibodies against AADC in 35 of 69 patients (51%) with APS I but in none of 138 patients with isolated IDDM or 91 healthy controls. Among the patients with APS I, anti-AADC antibodies were more often found in those with hepatitis (11/12, 92%), than in those without hepatitis (24/57, 42%) (P = 0.003). Similarly, of 15 patients with vitiligo, 12 (80%) had anti-AADC antibodies, compared with 23/54 (43%) without vitiligo (P = 0.021). Of the 9 APS I patients with IDDM, 5 had antibodies against both AADC and glutamate decarboxylase, 2 against AADC only, and 2 against glutamate decarboxylase only. Interestingly, AADC is present in relatively large amounts in the liver, where its function is unknown. Thus, an autoimmune reactivity against AADC may be involved in the pathogenesis of autoimmune chronic active hepatitis and vitiligo in APS I patients, whereas the role of AADC in the development of IDDM in these patients remains to be determined.