Peroxisomal acyl-coenzyme A (acyl-CoA) oxidase deficiency is an autosomal recessive inborn error of peroxisomal fatty acid oxidation due to a deficiency of straight-chain acyl-CoA oxidase (SCOX). The biochemical hallmark of this disorder is the accumulation of very long-chain fatty acids. Although some case reports and small series of patients have been published, a comprehensive overview of the clinical, biochemical, and mutational spectrum of this disorder is still lacking. For this reason, we report clinical information for a cohort of 22 patients with peroxisomal acyl-CoA oxidase deficiency and the results from biochemical and mutation analyses in fibroblasts of the patients. No clear genotype-phenotype correlation was observed. An intriguing mutation in the alternatively-spliced transcript encoding the isoform SCOX-exon 3II in a patient with normal expression of the transcript encoding the isoform SCOX-exon 3I, prompted us to characterize these two isoforms of human SCOX. The recombinant SCOX-exon 3I displayed activity toward medium-chain fatty acyl-CoAs and was not active with very long-chain fatty acyl-CoAs. In contrast, recombinant SCOX-exon 3II was capable of oxidizing a broad range of substrates, including very long-chain fatty acyl-CoAs. These results explain why this patient with a mutation in exon 3II of the ACOX1 gene, but with normal expression of exon 3I, was indistinguishable from other patients with peroxisomal acyl-CoA oxidase deficiency with respect to his clinical presentation and the biochemical abnormalities in his fibroblasts. Hum Mutat 28(9), 904-912, 2007.