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.