Medium-chain acyl-coenzyme A dehydrogenase deficiency is an autosomal recessive disorder of beta-oxidation of fatty acids manifested by episodic hypoglycemia, encephalopathy, apnea, and sudden death. Medical data were obtained on 120 patients with medium-chain acyl-coenzyme A dehydrogenase deficiency referred to Duke University Medical Center for biochemical testing. There were 55 male and 65 female subjects ranging from birth to 19 years of age; 118 subjects were white. Twenty-three children (19%) died before the diagnosis was made. Follow-up data were available in the 97 surviving patients for an average of 2.6 years after diagnosis. Psychodevelopmental data were collected on 73 patients older than 2 years of age. Unexpected morbidity included developmental and behavioral disability, chronic muscle weakness, failure to thrive, and cerebral palsy. We conclude that unidentified patients with this disorder have a significant risk of sudden death in early childhood and that survivors have a significant risk of developmental disability and chronic somatic illness.