Objective:To perform clinical and genetic testing and follow-up on two neonates to confirm the diagnosis of suspected medium chain acyl-coenzyme A dehydrogenase deficiency（MCADD）by newborn screening using tandem mass spectrometry（MS/MS）.Methods:Acylcarnitine profile was measured by liquid chromatography tandem mass spectrometry（LC-MS/MS）in peripheral blood and liver function. Urinary organic acid was tested by gas chromatography/mass spectrometry（GS/MS）. Medium chain acyl-Co A dehydrogenase gene（ACADM）of both patients and their parents was tested by high-throughput sequencing and Sanger sequencing.Results:（1）Both children’s acyl carnitine（C6-C10）was elevated.（2）Liver function was abnormal.（3）One patient’s glycerin and carboxy acid increased slightly

while the other’s showed normal.（4）Both children’s ACADM gene was compound heterozygotes. The genetic abnormality was inherited from the father and mother respectively

who were asymptomatic single mutant carriers. The compound heterozygous mutations resulted in abnormal activity of ACADM. Conclusion:The compound mutations of ACADM gene cause the decrease of MCAD activity and the patients ’abilities of decomposing the medium-chain fatty acid and then the abnormality of metabolites and liver fuction.