Helicobacter pylori causes a chronic infection in gastric mucosa, but its systemic effects are largely unknown. Our aim was to characterize the effect of H. pylori infection and gastric mucosal inflammation on the peripheral blood leukocyte count. An endoscopic series of 96 patients (40 men and 56 women), with a mean age of 62 years (range 49-80) was studied. Endoscopy with eight stepwise biopsies was performed and the occurrence of H. pylori was studied from sections stained with Warthin-Starry. The severity of inflammation in antral and body mucosa was estimated. The peripheral blood leukocyte count and differential count were determined by the automatic flow cytometric method. The total number of blood leukocytes and the numbers of lymphocytes and basophils were significantly increased in H. pylori-positive patients (N = 58), as compared with H. pylori-negative ones (N = 38). The total number of blood leukocytes correlated with the numbers of neutrophils, eosinophils, and mononuclear cells in the gastric mucosa. The number of basophils correlated with the number of mucosal neutrophils and mononuclear inflammatory cells. The results show that mucosal inflammation due to H. pylori infection is reflected in the amount of peripheral blood leukocytes. Basophilia suggests involvement of allergic mechanisms in H. pylori gastritis.