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ABSTRACT: Introduction & Objective: Methylphenidate is one of the basic drugs in treating ADHD. According to many clinical studies, the foreign form of methylphenidate (ritalin) is more efficient than the Iranian form of the drug (stimidate). This study aimed to compare the efficacy of stimidate and Ritalin in the treatment of attention deficit hyperactive disorder. Materials & Methods: In this double blind, randomized clinical trial, 200 children with attention deficit hyperactive disorder who referred to Isfahan psychiatric clinics were studied. For 100 of these patients ritalin was prescribed while others received stimidate. After 4 weeks, changes in severity of symptoms were evaluated with parental form of Conner's questionnaire. Results: After 4 weeks of treatment, the mean decrease of Conner's number in Ritalin group was 19.63±13.5 and in Stimidate group was 3.29±7.2. Ritalin had effectiveness in treatment of 83.3 percent of the patients ( 6 reduction in Conner's number), but Stimidate was only effective in treating 37.5 percent of the patients. Conclusion: This study showed that foreign methylphenidate (ritalin) is more effective than the Iranian form of the medicine (stimidate).