Abstract A 44-year-old woman had tender erythematous nodules in both the upper and lower extremities, headache, and fever during the course of propylthiouracil therapy for Graves' disease. Serologic tests showed high titers of antineutrophil cytoplasmic antibody (ANCA) against myeloperoxidase (MPO). A skin biopsy showed neutrophilic dermatitis consistent with Sweet's syndrome. After the cessation of propylthiouracil therapy and the administration of steroids, all her symptoms disappeared and the titer of antineutrophil cytoplasmic antibody against myeloperoxidase decreased. A causal relationship between propylthiouracil (PTU) therapy and Sweet's syndrome is suggested.