The results of a medium-chain triglyceride loading test in a patient with severe carnitine-acylcarnitine translocase deficiency clearly demonstrated impaired in vivo utilization of medium-chain triglycerides. The loading test was performed at the ages of 7 and 36 months. The diet was adjusted accordingly. The clinical course has been favourable and the child is now in very good condition at age 4 years. We conclude that the utilization of medium-chain triglycerides is only partial in carnitine-acylcarnitine translocase deficiency and cannot reasonably be considered an optimal source of energy for these patients. Careful adjustment of dietetic treatment may help to improve prognosis.