Background/aim: Autism is a neurodevelopmental disorder identified with higher frequency of serotonin abnormalities and gastrointestinal (GI) and sleep problems. This study aimed to evaluate the plasma levels of serotonin, GI symptoms, and sleep problems, and their relationship with autism severity in children with autism. Materials and methods: Thirty-five children with autism and 31 healthy subjects were studied. GI problems, sleep disorders, and severity of disorder were assessed. Plasma serotonin was determined using ELISA. Results: There was no significant association between GI problems and autism severity, but a significant positive correlation was seen between different indicators of sleep disorder and severity of autism. Plasma levels of serotonin were significantly higher in autistic children and a significant negative correlation was observed between plasma levels of serotonin and autism severity (r = -0.39, P = 0.02). Conclusion: Elevated plasma serotonin in autistic children and its negative correlation with disease severity may indicate involvement of the neurotransmitter in the neurophysiologic mechanism of autism.