A patient with a long history of exercise-unduced pain developed myoglobinuria and respiratory failure following extensive exercise (football). Although muscle histochemistry was normal, tissue oxidation of 14C-labeled palmitate was decreased, and muscle carnitine palmityltransferase (CPT) activity was one-tenth of normal. During fasting, his creatine kinase (CK) rose from 127 mu/ml to 278 mu/ml and blood ketones failed to exhibit a normal rise. Triglycerides were normal, as was fatty-acid mobilization. Prolonged exercise resulted in an inordinately increased CK with only moderate elevations in lactate. Treatment with medium-chain triglycerides did not alter his symptoms or improve exercise performance. Pain on exercise is a common complaint, but the occurrence of myoglobinuria points to a defect of energy metabolism; Screening for defects of fat utilization may be accomplished by the prolonged-exercise test, invitro oxidation of 14C-labeled substrates, and prolonged fasting.