A 40-year-old man had regular cyclic episodes of weight gain and eosinophilic myositis associated with hyperimmunoglobulin-E and hypereosinophilia for 9 years. During the episodes his body weight increased up to 10.8%; eosinophil counts reached 41.3 X 10(-9) cells/L; and IgE levels reached 18 000 IU/mL. These values changed regularly in a definite sequence relative to the clinical state. Attempts to document a parasitic cause were unsuccessful, and several courses of anthelmintic therapy were ineffective. An oral dose of prednisone, 10 mg/d, begun in July 1982 resulted in an immediate lessening of the severity of the episodes and a progressive lengthening of the cycle from 35 to 170 days. No further episodes have occurred since March 1984. The patient is fit and well on prednisone therapy, 12.5 mg on alternate days. This apparently unique syndrome has a benign course and is a cyclic disease involving skeletal muscle as the target organ.