Abstract Background Precise mechanisms underlying the pathophysiology of autism are currently unknown. Given the major role of glutamate in brain development, we have hypothesized that glutamatergic neurotransmission plays a role in the pathophysiology of autism. In this study, we studied whether amino acids (glutamate, glutamine, glycine, d-serine, and l-serine) related to glutamatergic neurotransmission are altered in serum of adult patients with autism. Methods We measured serum levels of amino acids in 18 male adult patients with autism and age-matched 19 male healthy subjects using high-performance liquid chromatography. Results Serum levels (mean = 89.2 μM, S.D. = 21.5) of glutamate in the patients with autism were significantly ( t = − 4.48, df = 35, p < 0.001) higher than those (mean = 61.1 μM, S.D. = 16.5) of normal controls. In contrast, serum levels of other amino acids (glutamine, glycine, d-serine, l-serine) in the patients with autism did not differ from those of normal controls. There was a positive correlation ( r = 0.523, p = 0.026) between serum glutamate levels and Autism Diagnostic Interview–Revised (ADI-R) social scores in patients. Conclusions The present study suggests that an abnormality in glutamatergic neurotransmission may play a role in the pathophysiology of autism.