We describe the clinical features and biochemical findings of two patients with long-chain 3-hy-droxyacyl-CoA dehydrogenase deficiency. Both children presented with an acute metabolic crisis. Both had hypo-glycemia and excreted even-chain unsubstituted dicarbox-ylic and 3-hydroxy-dicarboxylic acids in the urine. Measurement of the enzymes of fatty acid oxidation in cultured skin fibroblasts showed low activity of long-chain 3-hy-droxyacyl-CoA dehydrogenase, but normal activity of short-chain 3-hydroxyacyl-CoA dehydrogenase. The defect was further characterized by immunoprecipitating the short-chain enzyme using monospecific antibodies. It is probably inherited as an autosomal recessive trait, inasmuch as intermediate enzyme activity was found in the fibroblasts from the parents of one child.