The use of propylthiouracil (PTU) has been associated with various forms of vasculitis. We herein describe the case of a patient with Grave's disease who, after years of PTU therapy, developed a necrotizing vasculitis with anti-serine protease-3 antibodies. Despite treatment with corticosteroids and cyclophosphamide, the patient died of intra-alveolar hemorrhage secondary to her vasculitis. Based on the vessel size involved, the organ distribution of pathologic findings, and lack of granulomas, autopsy findings were felt to be more consistent with microscopic polyangiitis (MPA) than with her original clinical diagnosis of Wegener's granulomatosis. Her case satisfied both clinical and pathologic criteria for MPA. An MPA diagnosis is important to consider in similar clinical presentations because therapy may just need to be early withdrawal of an inciting drug, such as PTU, and the initiation of corticosteroids without cytotoxic therapy.