A cross-sectional study was conducted to evaluate adverse effects associated with risperidone in 45 children with autism spectrum disorders (ASD), aged 2-15 years, who were treated at Siriraj Hospital, Thailand, between the years 2006 and 2007. Adverse effects were assessed by parent interview, using a semistructure questionnaire, and medical records review. The mean ± SD age of the children at starting risperidone was 8.15 ± 2.98 years. The mean ± SD of risperidone dose was 0.94 ± 0.74 mg/day and the mean ± SD duration of treatment was 36.8 ± 27.8 months. Adverse effects were reported in 39 children (86.7%). Common adverse effects included increased appetite, somnolence, and rhinorrhea and most of the adverse effects were tolerable. Tardive dyskinesia or other serious adverse events were not found in this study. The child's mean ± SD weight gain was 4.18 ± 2.82 kg/year, which exceeded developmentally expected norms. The results from this study suggest that risperidone treatment in children with ASD is associated with frequent mild and tolerable adverse effects. However, excessive weight gain could be found to be a concerning adverse effect and weight monitoring is warranted when risperidone is being prescribed.