In the present paper two siblings are presented with clinical manifestations very similar to those of patients affected by neonatal adrenoleukodystrophy. In contrast to neonatal adrenoleukodystrophy patients, hepatic peroxisomes in these siblings were enlarged in size and not decreased in number. Accumulation of very-long-chain fatty acids (VLCFA) was associated with an isolated deficiency of the fatty acyl-CoA oxidase, the enzyme that catalyzes the first step of the peroxisomal beta-oxidation. Plasma levels of di- and trihydroxy-coprostanoic acid, phytanic acid, and pipecolic acid were normal; furthermore, acyl-CoA:dihydroxyacetone phosphate acyltransferase activity in cultured fibroblasts was also found to be normal. The clinical, biochemical, and cytochemical features found in these two siblings are compared with those seen in two other disorders characterized by the absence of a decreased number of hepatic peroxisomes and the presence of VLCFA: (1) pseudo-Zellweger syndrome (deficiency of peroxisomal thiolase activity) and (2) X-linked childhood adrenoleukodystrophy (deficiency of activation of lignoceric acid). Review of the different biochemical defects possible in very-long-chain fatty-acid oxidation reveals different clinical pictures of varying severity, depending on the level at which the biochemical defect occurs.