The immune system plays an important role in chronic gastroduodenitis. A group of 70 children with chronic gastroduodenitis were subjected to examinations, including 42 children with H. pylori infection (group I) and 28 children without H. pylori infection (group II). Applying the method of flow cytometry, the per cent values of peripheral blood lymphocytic subpopulations were determined before and after treatment. The obtained results of examinations in group I were compared with those obtained in group II and were also compared with the standards given by Zeman for healthy children. On the basis of analysis of own material it was found that in both studied groups before and after the treatment lower per cent values were observed of CD3 and CD8 T-cells, NK CD56 cells and higher value of CD4 to CD8 T-cell ratio was disclosed as compared to healthy children. Comparing the per cent values of lymphocytic subpopulations in children with and without H. pylori infection it was found that in group I a higher per cent of CD4 T-cells, lower per cent of CD8 T-cells and, therefore, higher value of CD4 to CD8 T-cell ratio were present.