Propylthiouracil (PTU) is recommended as a first-line antithyroid drug (ATD) during first trimester organogenesis in pregnancy because recent evidence suggests that methimazole (MMI) may be associated with congenital anomalies. However, PTU more commonly causes myeloperoxidase (MPO)-antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis, which usually occurs during prolonged treatment, compared with MMI. We report a case of MPO-ANCA-associated vasculitis in a 35-year-old woman with Graves'disease. Although her thyroid function could be maintained euthyroid by MMI, her ATD was switched to PTU because she wished to become pregnant. The patient presented with flu-like symptoms 8 days after starting PTU and developed hemoptysis and dyspnea at 22 days. Her MPO-ANCA titer was 21 ELISA units (EUs) before PTU treatment but increased to 259 EUs at 22 days after PTU treatment. Her clinical condition improved with the discontinuation of PTU and with immunosuppressive therapy. This case indicated that MPO-ANCA vasculitis occurred within several weeks after the initiation of PTU and that this side effect could be caused by the change from MMI to PTU. Thus, our clinical observation suggests that patients treated with PTU should be carefully monitored for MPO-ANCA titers and variable manifestations of MPO-ANCA-associated vasculitis regardless of the period of administration.