This study aimed to see whether measuring serum glutamate in children with autism spectrum disorder (ASD) could provide a biological marker that could allow early intervention.Serum glutamate was measured in 30 patients aged 3-10 years presenting with ASD to the Abou El Reesh Hospitals, Cairo University, Egypt and 30 matched controls without ASD in 2015. The Vineland Social Maturity Scale was applied to assess social competence, self- help skills and adaptive behaviour in both groups. The severity of autism was measured with the Childhood Autism Rating Scale test.The patients' group showed higher mean values of serum glutamate (5.888) than the control group (2.521) and the statistical difference was significant (p = 0.00021). There was no significant difference (p = 0.151) in the serum level of glutamate between patients receiving 1-2 mg of risperidone (6.519 ± 2.851) and those who were free from any medication for at least six weeks (5.157 ± 2.184).We found higher levels of serum glutamate in subjects with ASD and this might reflect altered glutamatergic neurotransmission which may aid early ASD detection. Further investigations are needed with a large number of participants to further clarify the possibility of using glutamate as a biomarker for ASD.