Palmitoyl-CoA dehydrogenase activity in skin fibroblasts from seven patients with unidentified defects of fatty acid oxidation was measured in the presence and absence of antibodies against medium-chain, long-chain, and very-long-chain acyl-CoA dehydrogenases (VLCAD). Two of the patients, 4-5 month old boys, were found to have a novel disease, VLCAD deficiency, as judged from the results of very low palmitoyl-CoA dehydrogenase activity and the lack of immunoreactivity toward antibody raised to purified VLCAD.