Serotonin is necessary for normal fetal brain development. Administration of serotonin inhibitors to pregnant rats results in offspring with abnormal behaviors, brain morphology, and serotonin receptor numbers. Low maternal plasma serotonin may contribute to abnormal brain development in autism. In this study, plasma serotonin levels in autism mothers and control mothers of typically developing children were compared, and plasma serotonin levels in children with autism (n = 17) and their family members were measured. Plasma serotonin levels in autism mothers were significantly lower than in mothers of normal children (P = 0.002). Plasma serotonin levels correlated between autism mothers and their children, but differed between autistic children and their fathers (P = 0.028) and siblings (P = 0.063). Low maternal plasma serotonin may be a risk factor for autism through effects on fetal brain development.