A progressive demyelinating cerebral disorder is described in a normally-appearing female infant with neonatal seizures, progressive psychomotor deterioration, deafness, retinopathy, peripheral neuropathy and loss of myelin observed on magnetic resonance imaging (MRI) scanning. MRI also showed the absence of macroscopic neocortical dysplasia which is usually found in Zellweger syndrome (ZS). Adrenal cortical function was normal. The patient died at the age of 37 months. Extensive biochemical investigations of peroxisomal functions in the patient revealed an impairment of peroxisomal beta-oxidation resulting in elevated levels of very long (greater than C22) chain fatty acids in plasma and fibroblasts. Moreover, elevated plasma levels of intermediates of bile acid biosynthesis such as tri- and dihydroxycholestanoic acid were found. Other peroxisomal functions were normal. Immunoblotting of the peroxisomal beta-oxidation enzyme proteins in liver from the patient revealed normal responses with antisera against acyl-CoA oxidase, bifunctional protein and thiolase respectively. From these data we conclude that the patient had a deficiency of a single peroxisomal beta-oxidation enzyme at the level of either the bifunctional protein or peroxisomal thiolase with retained immunoreactivity against these enzymes.