Attention-deficit hyperactivity disorder (ADHD) is a common psychiatric disorder among preschool children but the number of controlled clinical trials regarding psychopharmacological treatment in this age group is limited. The aim of this study was to compare methylphenidate with the combination of methylphenidate and risperidone in preschool children with ADHD. Forty-two preschool children, aged 3-6 years, diagnosed with ADHD by a child and adolescent psychiatrist according to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition-Text Revision criteria, were enrolled in a 6-week, single-blind clinical trial and administered with methylphenidate (5-30 mg/dl) or the combination of methylphenidate and risperidone (0.25-2 mg/dl) in Iran. Treatment outcomes were assessed using the Conners' Rating Scale and Clinical Global Impression (CGI) Scale at baseline and 3 and 6 weeks after starting the drugs administration. Side effects were rated by a checklist and body weight was measured at each visit. There were no significant differences between the two protocols in Parent Conners' Rating Scale scores ( > 0.05) and CGI scores ( > 0.05). Both groups showed a significant improvement in ADHD symptoms over the 6 weeks of treatment for Parent Conners' Rating Scale ( < 0.001). The combination group used significantly lower doses of methylphenidate compared to the other group ( = 0.002). The most common adverse effects were anorexia (21.7%) and daytime drowsiness (17.4%) in combination treatment group and insomnia (33.3%) and anorexia (25%) in methylphenidate group. Risperidone and methylphenidate may be effective and well tolerated in preschool children with ADHD, and adding risperidone to methylphenidate may decrease the occurrence of some side effects of methylphenidate such as insomnia and anorexia and lower the dose of methylphenidate may be needed to control symptoms.