Patients with Graves' disease (n=61) treated wth propylthiouracil (PTU) or thiamazole (MMI) were studied retrospectively to investigate differences in the prevalence of anti-myeloperoxidase anti-neutrophil cytoplasmic antibodies (MPO-ANCA) in relation to treatment with anti-thyroid drugs. The patients were divided into two groups: PTU-treated group (n=32) and MMI-treated group (n=29). There were no significant differences between the two groups in terms of age, gender distribution, or duration of treatment. In the PTU group, 8/32 (25%) patients were positive for MPO-ANCA, whereas in the MMI group, 1/29 (3.4%) patients were positive. There were no significant differences in age, duration, or dosage between the MPO-ANCA positive and negative patients. Most of the MPO-ANCA positive patients were asymptomatic, except for two patients in whom rheumatic arthritis or membranous glomerulonephritis developed. None of the MPO-ANCA positive patients were diagnosed as having classical ANCA-associated vasculitis. Thus, there is a high frequency of MPO-ANCA in patients with Graves' disease treated with PTU, compared with patients treated with MMI, although classical ANCA-associated vasculitis develops in only a few MPO-ANCA positive patients.