ntroduction Attention Deficit Hyperactivity Disorder (ADHD) is relatively common in children and has many co morbidities. Some studies have shown that co morbidities especially anxiety disorders may affect the response to stimulant medication. The object of this study was to investigate the relation of children\'s response to methyl-phenidate with their co morbid psychiatric disorders. Method The sample included fifty 6 to 12 years old children with ADHD referred to Dr.Sheikh clinic of child psychiatry in Mashhad (north-east of Iran). The diagnoses were made by a child psychiatrist based on the DSM-IV -TR criteria, using clinical interview and K-SADS. ADHD children received methyl-phenidate for 4 weeks. ADHD- rating scales were completed by mothers and teachers of children before the treatment and after 4 weeks. Data were analyzed by SPSS -16 and statistical tests including chi-square and t-test. Results The mean age of ADHD children was 8.7 years old. Most of the patients (74%) were boys. In 32 patients (64%) at least one co morbid psychiatric disorder was found. Anxiety disorders were the most common co morbidity of ADHD (in 56% of patients). Co morbid disorders did not have a significant affect on response to medication in ADHD children (P>0.05). Conclusion In the present study disruptive behavior and anxiety disorders were the most common co morbidities in ADHD children. Presence of co morbid disorders including anxiety disorders did not change the response to methyl -phenidate.

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