A 21-year-old man had recurrent myoglobinuria; his 28-year-old sister had symptoms of fatigability. During prolonged fasting, serum free fatty acid rose in both siblings, but only the sister produced ketone bodies and had elevated creatine phosphokinase activity. Carnitine palmityl transferase (CPT) activity was less than 30% of normal in muscle and platelets. Liver biopsy disclosed a low level of the enzyme in the brother. The parents had intermediate levels of the enzyme in platelets. CPT deficiency seems to have an autosomal-recessive pattern of inheritance and a variable phenotypic expression.