Objective: To analyze the heterogeneity of in frequent symptoms among Chinese children with methylmalonicaciduria (MMA)originated from a mixed genetic background.Subjects and Methods:We investigated 26 Chinese patients (17 males and 9 females) diagnosed by elevated urinary MMA, elevatedserum C3, C3/C2 ratio and decreased serum free carnitine.Genetic diagnoses of these children were made by sequencing of MUT, MMAA, MMAB, MMACHC, MMADHC, LMBRD1, MCEE, SUCLG1,SUCLG2and ABCD4 gene.The clinical and biochemical features were analyzed.Results:We identified a considerable variation between clinical manifestations of these children.Symptoms of MMA varied from mild vomiting to severe encephalopathy andthe age onset from 3 days to 13 years.Less frequent symptoms (<10%) such as hydrocephalus, isolated pulmonary hypertension, peripheral neuropathy and diabetic ketoacidosis were found as the first signs in MMA patients.Mutatios in MUT, MMACHC and MMADHC gene were identified in 22 patients.Among these mutations, two novel missense mutations in MUT gene (c.1540C>A and c.505G>T) were identified.In 2 patients with isolated pulmonaryhypertension, we found the same mutation sites and so that in 2 patients with hydrocephalus (Table 3).Conclusions: The heterogeneous clinical manifestation identified among our Chinese patients with MMA indicated that the genetic profiles of MMA patients vary from ethnics and there were phenotype-genotype correlations according to our genetic sequencing results.