A 31-year-old man had a nine-year history of exercise-induced cramps and muscle pain without myoglobinuria. Results of laboratory investigations differentiated his condition from the known disorders of carbohydrate and lipid metabolism. Light and electron microscopic examination of a muscle biopsy specimen showed tubular aggregates confined to type II fibers. Although the relationship of tubular aggregates to muscle cramps is uncertain, this association has been described previously and may be significant.