Adenotonsillar hypertrophy and obstructive sleep disordered breathing can lead to attention deficit/hyperactivity disorder (ADHD). The purpose of this study was to evaluate effect of adenotonsillectomy on improvement of ADHD symptoms in a quasi-experimental (before and after) study. The efficacy of adenotonsillectomy on improvement of ADHD symptoms of 35 children aged 5-12 years with adenotonsillar hypertrophy and ADHD was evaluated six months after surgery. Diagnosis of ADHD was based on the DSM-IV criteria in three subtypes (predominantly inattentive type, predominantly hyperactive-impulsive type and combined type). Seventeen boys (49%) and eighteen girls (51%) with mean (± SD) age of 7.4 ± 3.8 years (range: 1-10 years) were evaluated. Frequency of combined type of ADHD decreased significantly six months after adenotonsillectomy (54.3% versus 22.9%, P=0.003). ADHD inattention score (2.26 ± 1.93 versus 0.96 ± 0.45, P=0.005), hyperactivity score (4.23 ±3.57 versus 3.57 ±8, P=0.03) as well as ADHD combined score (9.66 ±2.58 versus 7.2 ±3.67, P=0.0001) improved significantly after surgery. Upper air way obstruction due to adenotonsillar hypertrophy might be an important and treatable cause of ADHD and should be considered in evaluation of affected children. Adenotonsillectomy in these children is associated with improvements in ADHD symptoms.