Background: G.lamblia is a common worldwide parasitic infection which colonizes the upper part of small intestine causing giardiasis. Aim of the work: This study aimed to detect gastric giardiasis in children receiving antacid drugs and undergoing upper endoscopy at El Mouniera Children Hospital. In addition to study the effect of H.pylori on colonization of gastric mucosa by G.lamblia trophozoites and assess the associated histopathological changes in gastric and duodenal mucosa. Subjects&Methods: 100 patients (1to15 years of age) of both sexes (60% male &40% female) were classified into group I includes 70 patients receiving antacid drugs and group II includes 30 patients not receiving antacid drugs. All patients were subjected to history taking, clinical examination, stool analysis, endoscopy, biopsy taking and detection of Giardia coproantigen in stool by ELISA. Biopsies were stained with Haematoxylin & Eosin, Giemsa and Masson trichrome stains. Results: Abdominal pain, flatulence, vomiting and diarrhea constituted the most frequent complains among studied population. In 58% of all cases endoscopy was normal while endoscopic findings included distal oesophagitis (20%), esophageal varices (14 %), gastritis (8%), duodenitis (8%) and gastric ulcer (2%). G.lamblia trophozoite was detected in 5.7% of cases combined with H.pylori in group I with significant association (p value=0.009) and associated with chronic gastritis (75%), lymphocytic gastritis (50%), metaplasia (50%) and atrophic gastritis (25%). In the all cases, G.lamblia was detected in 16% of stool samples, 11% of duodenal biopsies and 20 % of antigen detection by ELISA. Intestinal giardiais (11%) was associated with chronic inflammatory infiltrate (90.9 % %), flattening and shortening of villi (63.6%) and duodenitis (18.2%). H.pylori was detected in 43 % of the all cases and associated with chronic gastritis (34.8 %), metaplasia (34.8 %), lymphocytic gastritis (32.5 %), erosive gastritis (11.6%) and atrophic gastritis (9.3 %). Conclusions: Suppression of gastric acid secretion encourages colonization of gastric mucosa by G.lamblia trophozoite especially in the presence of H.pylori, chronic gastritis, metaplasia or lymphocytic gastritis.