Sweet's syndrome is defined as acute febrile neutrophilic dermatosis. Characteristic features are fever; peripheral neutrophilia; and painful cutaneous nodules and plaques on the face, neck, trunk, and limbs. Biopsy specimens of these lesions show a mature neutrophilic infiltrate of the dermis. Vasculitis is absent. Sweet's syndrome is associated with malignancy in approximately 20% of reported cases. The pathogenesis is unknown. The authors describe Sweet's syndrome in a 39-year-old man 5 weeks after splenic irradiation for chronic myelogenous leukemia. Treatment with parenteral corticosteroids resulted in dramatic improvement of the patient's condition. The authors discuss the diagnosis of Sweet's syndrome and the fact that it is thought to be cytokine-induced.