Six patients with carnitine palmitoyltransferase (CPT) deficiency were diagnosed. Five were males aged 12-48 years and one was a 7-year-old girl. Severe myoglobinuria with renal shut-down led to the diagnosis in four, the main cause of myoglobinuria being prolonged exertion. Other precipitating factors included febrile illnesses, fasting and sleep deprivation. Between attacks, EMG, serum creatine kinase (CK) activity and the response of serum lactate to ischemic exercise were normal in all patients. The diagnosis of CPT deficiency was based on assay of the enzyme by the isotope exchange assay in leukocyte, muscle or cultured skin fibroblast homogenates. CPT activity assayed by the forward assay under standard conditions was normal. CPT deficiency seems to be a common cause of exercise-induced myoglobinuria. Prompt diagnosis can lead to avoidance of risk factors and prevention of rhabdomyolysis and its consequences.