Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is a frequent and sometimes fatal inherited metabolic disorder of fatty acid β-oxidation. A eukaryotic expression system was used to study naturally occurring mutations in MCAD. The 1263 nucleotide coding region of human MCAD cDNA was inserted downstream of the SV40 early promoter for high-level expression in Chinese hamster ovary cells. Both normal MCAD cDNA and a mutant MCAD cDNA containing the common, disease-causing A to G transition at position 985 (A985G), which alters a lysine to a glutamic acid (K304E), were inserted into expression vectors. Transient transfection of Chinese hamster ovary cells was performed with the expression constructs. The steady state level of expressed normal MCAD protein antigen was substantially higher (5-fold) than the expressed mutant protein. The MCAD enzymatic activity in protein extracts from cells containing the expressed normal MCAD cDNA was also much higher (6-fold) than the activity in cells expressing the mutant MCAD. Therefore, these data confirm that the common K304E mutation causes MCAD deficiency primarily by decreased protein stability rather than reduction of catalytic activity and, in fact, demonstrate that the K304E mutant protein has a similar sp act against octanoyl CoA substrate as the normal protein.