Early in the course of myeloblastic leukemia a patient concurrently developed febrile neutrophilic dermatosis and sterile acute myositis. The dermatitis and myositis were unresponsive to antibiotic therapy but remitted within a few days of institution of steroid treatment. The patient died of myocardial infarction. At autopsy the dermis was normal. Previously effected muscles were scarred. The overlying fascia and subcutaneous septa were fibrotically thickened. In addition, segmental acute aortitis was detected. Acute myositis and aortitis may reflect further organ manifestations of the Sweet's reactivity pattern. It is proposed that Sweet's myositis and dermatitis may evolve into a fibrosing myositis and panniculitis.