Mucopolysaccharidosis IIIA (MPS IIIA) is a lysosomal storage disorder characterized by progressive mental deterioration and severe behavioral problems. We conducted an open-label, crossover study of the efficacy and safety of Risperidone on behavioral disorder in children with MPS IIIA. A total of 12 patients (5.5+/-2.2 years) with enzymatic diagnosis of MPS IIIA were randomly assigned to receive Risperidone (0.125-2mg/d) for 6 months. The hyperactivity and disruptive behavior level of children before and after treatment was evaluated regarding the scores from Turgay DSM IV Based Child and Adolescent Behavior Disorders Screening and Rating Scale (T-DSM-IV-S). Clinic Global Impression Scale - Severity (CGIS-S) was used for all cases for determining the psychiatric disorder severity. The anxiety and depression levels of mothers before and after treatment were evaluated using Hamilton Anxiety Scale (HAM-A) and Beck Depression Inventory (BDI). The adverse effects were evaluated by monitoring weight, serum prolactin, glucose and lipid levels. The response to the treatment was measured by decrease in values of CGI-S (from 6+/-1.12 to 2.91+/-0.66, p=0.001). According to T-DSM-IV-S scores the best improvement was observed in hyperactivity scores (16.25+/-8.57/11.58+/-7.26, p=0.001), followed by opposition/defiance (6.66+/-5.92/5.08+/-4.88, p=0.032), and conduct disorder scores (1.00+/-1.85/0.41+/-.99, p=0.67). No clinically relevant elevations in weight and serum prolactin, glucose or lipid levels have been documented (p>0.05). There was a significant decrease in anxiety and depression scores of mothers (HAM-A: 20.33+/-8.28/17.91+/-6.89, BDI: 23.58+/-7.14/20.5+/-5.93, p<0.001). To our knowledge, research on the pharmacological treatment of MPS IIIA with Risperidone has not been reported. According to our data, Risperidone appeared to be safe and effective in MPS IIIA patients. 2008 Elsevier B.V. All rights reserved.