Both genetic and environmental factors affect the risk of orofacial clefts. The present meta-analysis aimed to evaluate the association between methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism and risk of nonsyndromic cleft lip/palate (NSCL/P) in cases-control studies. The PubMed/ Medline, Scopus, Web of Science, and Cochrane Library databases were searched up to April 2019 with no restrictions. The odds ratios (ORs) and 95% confidence intervals (CIs) in all analyses were calculated by Review Manager 5.3 software. The funnel plot analysis was carried out by the Comprehensive Meta-Analysis version 2.0 software. Subgroup analysis, meta-regression, and sensitivity analysis were performed for the pooled analyses. Thirty-one studies reviewed in this meta-analysis included 4710 NSCL/P patients and 7271 controls. There was no significant association between MTHFR C677T polymorphism and NSCL/P susceptibility related to allelic model (OR = 1.04; P = 0.49), homozygote model (OR = 1.11; P = 0.35), heterozygote model (OR = 0.99; P = 0.91), dominant model (OR = 1.00; p = 0.96), or recessive model (OR = 1.08; P = 0.23). There was no significant association between MTHFR C677T polymorphism and NSCL/P susceptibility based on the ethnicity or the source of cases. There was a significant linear relationship between the year of publication and log ORs for the allele model. The results of the present meta-analysis failed to show an association between MTHFR C677T polymorphism and NSCL/P susceptibility. The subgroup analyses based on the ethnicity and the source of cases further confirmed this result.Non-syndromic cleft lip/palate (NSCL/P) is a common birth defect worldwide 1 . In low-and middle-income countries, around 1/730 children is born with cleft lip/palate 2 . A multifactorial model of genetic inheritance has been recommended for NSCL/P based on the interaction of genetic and environmental factors 1 . Several lines of evidence have proven a significant association between polymorphism of genes connected to folate metabolism and increased risk of orofacial clefts 3 . Among genes related to folate metabolism, 5,10-methylenetetrahydrofolate reductase (MTHFR) reportedly has the highest association with NSCL/P. This gene is located on chromosome 1 at 1p36.3 and translates to MTHFR enzyme that catalyzes the conversion of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, a cosubstrate for homocysteine remethylation to methionine 4,5 . MTHFR is a fundamental enzyme in folate metabolism and DNA synthesis, and MTHFR rs1801133 (C677T) is one of the most common polymorphisms which diminishes the enzyme activity 6-8 . Regulation of MTHFR activity is pivotal to maintain optimal cellular levels of methionine and S-adenosylmethionine 5 . Folate supplementation or its adequate dietary intake during pregnancy has been shown to prevent or decrease NSCL/P susceptibility 9 . Nutritional factors, such as the adequacy of folic acid in the mother's diet, are clearly important, but other potential disturbances in ovulation or ...