THE AIM OF THE STUDY was to assess the esophageal mucous membrane in children with pathological acid gastroesophageal reflux (GER) primary and secondary to cow's milk allergy and (or) food allergy (CMA/FA), dependent on the duration of the disease (prospective study). METHODS 264 children of both sexes suspected of GER were enrolled in the study. The age of examined children was 1.5-102 months (mean age 20.78+/-17.23 moths). Pathological acid GER was confirmed in 138 children (52.3%) with pH-monitoring. Taking into consideration complex differential diagnosis, including elimination of noxious nutrient and oral food challenge test (open or blind study), acid GER was differentiated into primary and secondary. Children were assigned into study groups 1 and 2. Group 1: 76 patients (28.8%) aged 4-102 months (mean age 25.2+/-27.28 months) with primary GER. Group 2: 62 patients (23.5%) aged 4-74 months (mean age 21.53+/-17.79 months) with GER secondary to CMA/FA. Endoscopic examination of the upper gastrointestinal tract was performed in 138 children: preliminary and control examination (after one year and two years of GERD diagnosis. RESULTS Esophagitis of different intensity (second - fourth degree) in macroscopic assessment was diagnosed in 25 children (32.9%) with group 1 and in 29 children (46.8%) with group 2 in preliminary study. Esophagitis was still present in macroscopic assessment after one year of conservative treatment in 13 children (17.1%) from group 1 and in 12 children (19.4%) from group 2. After two years of clinical observation and periodically administered conservative treatment mild esophagitis still persisted in 3 children (3.9%) from group 1, and in 3 children (4.8%) from group 2. CONCLUSIONS Esophagitis of various intensity (second - fourth degree) was diagnosed in preliminary endoscopic examination in about 33% of children with group 1 and 47% of children with group 2.