Since very long-chain fatty acids with a chain length of 24 carbons or more are known to accumulate in tissues and body fluids from patients with the cerebrohepato-renal (Zellweger) syndrome, infantile Refsum disease, neonatal adrenoleukodystrophy and X-linked adrenoleukodystrophy, we studied very long-chain fatty acid oxidation in cultured skin fibroblasts from these patients. In this paper, we report that in accordance with earlier results the first step in the beta-oxidation of the very long-chain fatty acid lignoceric acid (C24:0) primarily occurs in peroxisomes in control human skin fibroblasts. Furthermore, it was found that peroxisomal lignoceric acid beta-oxidation was strongly deficient in fibroblasts from patients with Zellweger syndrome, infantile Refsum disease, neonatal and X-linked adrenoleukodystrophy, which explains for the accumulation of very long-chain fatty acids in all four disease entities. In Zellweger syndrome, infantile Refsum disease and neonatal adrenoleukodystrophy the impairment in peroxisomal very long-chain fatty acid beta-oxidation is probably caused by a strong deficiency of all peroxisomal beta-oxidation enzyme proteins due to a deficiency of peroxisomes.