Objective　To explore the clinical and gene variation characteristics of methionine adenosyltransferase Ⅰ/Ⅲ deficiency. Methods　The clinical data and gene detection of methionine adenosyltransferase Ⅰ/Ⅲ deficiency in 5 newborns found by tandem mass spectrometry in neonatal screening were retrospectively analyzed. Results　In the 220000 newborns screened, 5 cases of methionine adenosyltransferase Ⅰ/Ⅲ deficiency were found and an incidence rate was 1/44000. In the 5 newborns, the concentrations of methionine were 70~150 μmol/L in 3 newborns, among whom 2 were autosomal dominant inheritance and one was autosomal recessive inheritance and all of them had a good prognosis. The blood methionine concentrations of the other 2 newborns were continuously greater than 500 μmol/L and they were autosomal recessive inheritance. These 2 newborns were treated with special diet. During follow-up, 1 patient had abnormal cranial magnetic resonance and abnormal liver function, and 1 patient had microdeletion syndrome and developmental retardation. Conclusion　Methionine adenosyltransferase Ⅰ/Ⅲ deficiency can be diagnosed early by tandem mass spectrometry combined with gene detection and the disease requires long-term follow-up.