Five patients aged 7 to 21 months are described who developed attacks of coma after a short prodromal illness with diarrhea or vomiting or both. Four had concomitant hypoglycemia, and all had hypoketonemia, with excessive urinary excretion of medium-chain dicarboxylic acids, medium-chain (omega-1)-hydroxyacids, suberylglycine, hexanoylglycine, and octanoylcarnitine. All patients accumulated octanoic acid, decanoic acid, and cis-4-decenoic acid in plasma. Fibroblasts from three patients showed a decreased rate of octanoate oxidation (10%, 12%, and 29% of control values, respectively). These findings suggest a deficiency of medium-chain acyl-CoA dehydrogenase, most probably an autosomal recessive inherited metabolic disorder. Two of the patients died during an acute attack, and a third had severe neurologic sequelae; the two remaining patients recovered. Plasma free carnitine levels were low, but total carnitine was normal. The three surviving patients underwent a fasting test, which did not lead to hypoglycemia, although hypoketonemia, dicarboxylic aciduria, and excessive mobilization of fatty acids did occur. The surviving patients were maintained on frequent carbohydrate-enriched meals.