It is not known if omeprazole possesses any action on immune system. Therefore, we examined the effect of omeprazole on parameters of cellular immunity [T-cell subsets-CD3+, CD4+, CD(8+)- and HLA-DR expression on peripheral blood lymphocytes (PBLs)] and on function of peripheral blood monocyte-macrophages (PBMMs) [random migration (RM), directed migration (DM), phagocytosis index (P-I) and HLA-DR expression] in 13 duodenal ulcer patients before and during 3-mo omeprazole treatment. The number of T-cell subsets varied at pretreatment values (p > 0.05), whereas the percentage of HLA-DR positive PBLs increased significantly after 3-mo therapy (p < 0.001). On the other hand, all studied parameters concerning PBMMs (RM, DM, P-I and HLA-DR expression) increased significantly after 3-mo therapy (p < 0.001, p < 0.001, p < 0.003, p < 0.001, respectively vs. baseline values). In conclusion, omeprazole exerts an immunopotentiating effect on functions of PBMMs and may also influence T-cell function. These effects can be considered as an advantage of omeprazole in long-term treated patients with peptic ulcer disease.