Background: It is believed that environmental and genetic factors may be responsible for autism. Methylenetetrahydrofolate reductase (MTHFR) and its gene polymorphisms have been shown to be implicated as risk factors in autism.

Objectives: To analyze MTHFR C677T polymorphism (rs1801133) in autistic patients.

Materials and Methods: This study was carried out in 2014 and 2015 in northern Iran. One hundred and seventy-one male autistic patients and 198 healthy males were included in this study. Autism was diagnosed according to the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV) criteria. Each autism spectrum disorder (ASD) patient was also evaluated by the Childhood Autism Rating Scale (CARS). All participants were tested for C677T polymorphism by using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method.

Statistical analyzes were performed using MedCalc version 12.1 by the χ² test and logistic regression model. A value of p<0.05 was considered statistically significant.

Results: The mean age±SD of case and control groups was 13.5±2.7 and 15.6±3.7 years. The mean±SD of CARS score was 36.2±1.7.

The genotype frequencies of CC, TC, and TT in children with autism were 50.9%, 45.6%, and 3.5%, respectively, and in control group were 54.5%, 44.0%, and 1.5%, respectively (p>0.05). The allele frequencies of C and T in children with autism were 73.0% and 27.0%, and in control group were 76.0% and 24.0%, respectively (p>0.05).

Conclusion: MTHFR C677T polymorphism is not associated with autism in a population in the north of Iran.