The aim of the present study was to determine the effects of adenotonsillectomy on height, weight and body mass index (BMI) in children under 12 years old, with or without airway obstruction and evaluation of the risk of overweight in them. In this case-control study, 120 children with the age of 2-12 years old were studied; 60 children as case group who underwent adenotonsillectomy and 60 healthy children as control group. After collecting the data related to appetite status and sleep breathing disorder of the case group, height, weight and BMI have been measured for all children in two stages; preoperatively and 6 months later. Also in the case group, BMI percentiles, pre and postoperatively have been calculated. Patients with Low appetite in the initiation and at the end of the study in the case group were 80% and 8.3% respectively (P=0.01). Mean of height, weight and BMI variation after 6 months were significantly different between case and control groups (P<0.05). BMI percentiles in the case group preoperatively were: 20% underweight, 67% healthy weight, 10% at risk of over weight, 3% over weight. Postoperatively, after 6 months BMI percentiles in order of above frequency were: 10%, 57%, 22% and 11% (P=0.02). Analysis of the results showed that adenotonsillectomy can lead to increase of height, weight, BMI and appetite not only in the children with low weight due to airway obstruction but also in the normal weight and over weight children. Therefore risk of overweight should be mentioned as a probable undesirable outcome of adenotonsillectomy.