In this paper we report the identification of a new disorder of mitochondrial fatty acid β-oxidation in a patient which presented with clear manifestations of a mitochondrial β-oxidation disorder. Subsequent studies in fibroblasts revealed an impairment in palmitate β-oxidation and in addition, a combined deficiency of long-chain enoyl-CoA hydratase, longchain 3-hydroxyacyl-CoA-dehydrogenase and long-chain 3-oxoacyl-CoA thiolase. The recent identification of a multifunctional, membrane-bound 9-oxidation enzyme protein catalyzing all these three enzyme activities (Carpenter et al. (1992) Biochem. Biophys. Res. Commun. 183, 443–448; Uchida et al. (1992) J. Biol. Chem. 267, 1034–1041) suggested an underlying basis for this peculiar combination of three enzyme deficiencies. We show by means of size-exclusion chromatography that there is, indeed, a deficiency of the multifunctional β-oxidation enzyme protein in this patient.