The research goal is to investigate age and sex influence on formation of gastroesophageal reflux disease in children with chronic gastroduodenitis. Material and Methods. Features of acidity in esophagus and cardial part of stomach was studied in 175 children aged 4 to 17 years with chronic gastroduodenitis by pH-monitoring. pH-monitoring was carried by «Gastroscan-24». Age and sex characteristics have been identified on examination results. Results. Children of preschool age experience the most unfavorable reflux from the stomach to the esophagus. Boys experience the pathological reflux more severely. Conclusions: Effectiveness of esophageal clearance is lower in children of preschool age. Pathological reflux progresses are more favorably in a standing position, especially in children of preschool age. Pathological reflux occurs more often in the supine position and has a longer and more aggressive course in preschool age boys