Presence of Helicobacter Pylori in the human stomach is associated with increased risk of distal gastric adenocarcinoma and gastric B-cell mucosa associated lymphoid tissue lymphoma. Several studies have shown increased cell proliferation in human carrying Helicobacter infection. After eradication therapy, increased proliferation returns to normal levels, which suggests that Helicobacter Pylori or the associated inflammatory response is responsible for the increased proliferation observed. Because of the increasing realization that cell turn over is dependent not only on proliferation but also on apoptotic cell loss, and because it is now appreciated that many pathogenic bacteria are capable of interacting with the apoptotic program of epithelial cells, the effect of Helicobacter Pylori on gastric epithelial cell apoptosis also has been recently investigated. The presence of Helicobacter Pylori has been associated with a 2-5 fold increase in gastric epithelial apoptosis in vivo that returns to normal levels after eradication of the organism in most studies. However in other studies, apoptosis was reported as unchanged or even decreased in the presence of Helicobacter Pylori. Aims of the study To study the immunohistochemical expression of P53, Bcl2, as apoptosis makers and ki67 and PCNA as proliferative markers in the gastric mucosa of patients infected with cagA positive strain of Helicobacter Pylori demonstrated by in situ hybridization. Patients, materials and methods The study was prospectively designed. A total of 150 adult patients presented with dyspeptic symptoms referred to the OGD (esophagogastroduodenoscopy) unit at Al-Kadhimiya teaching Hospital at Baghdad for upper endoscopy were included in the study; with an age range of 19-70 years (median 40 years) between June 2009 and March 2010 . Fifty one patients were excluded from the study because the biopsy material obtained during OGD was insufficient for the application of immunohistochemical and in situ hybridization studies. Three tissue biopsies were obtained, two from the antrum and one from the corpus. Rapid urease test was performed on one of the antral biopsies. The other biopsy specimens were paraffin embedded and processed. One section from each block was stained by H&E to study the histopathological features and grading of gastritis was done according to the updated Sydney system. One section was stained by modified Giemsa stain for better identification of H. pylori microorganisms. One section was used for In situ hybridization method to identify Cag-A starin H. pylori. Four sections were stained Immunohistochemically for Ki-67, PCNA, Bcl2 and P53. Results Sixty nine patients (69.69 %) were infected by H. pylori. Forty four patients had H. pylori cagA positive starin. Atrophy of gastric mucosa was present in 14 (14.14 %) patients. Intestinal metaplasia was present in 8 (8.08%) patients. The degree of atrophy was significantly higher in H. pylori gastritis than non-H. pylori gastritis (p=0.007). The degree of intestinal metaplasia was not significantly different in H. pylori gastritis than non-H. pylori gastritis (p=0.052). The activity of inflammation in the presence of cagA strain was significantly higher than that in the absence of cagA strain (p<0.001). The degree of atrophy was significantly higher in cagA H. pylori gastritis than non-cagA H. pylori gastritis (p=0.041). The degree of intestinal metaplasia was significantly higher in cagA H. pylori gastritis than non-cagA H. pylori gastritis (p=0.023). PCNA labeling index (LI) of the gastric glands was significantly higher in presence of atrophic alterations (p <0.001) and intestinal metaplais (p <0.001). The PCNA LI in the gastric mucosa was significantly higher in in cagA strain H. pylori positive gastritis than cagA strain H. pylori negative gastritis (p<0.001). Ki67 LI was significantly higher in gastric mucosa in the presence of atrophy than that of non-atrophic gastric corpus mucosa (p < 0.001). Ki67 LI was significantly higher in gastric mucosa in the presence of intestinal metaplasia than that of gastric mucosa without intestinal metaplasia (p < 0.05). Ki67 LI was significantly higher in gastric mucosa in the presence of cagA starin H. pylori than that of gastric mucosa in the absence of cagA starin H. pylori (p <0.05). Bcl2 expression was not significantly higher in H. pylori gastritis than non-H. pylori gastritis (p= 0.101). Bcl2 expression was significantly higher in the presence of atrophy (p<0.001). Bcl2 expression was significantly higher in the presence of intestinal metaplasia (p<0.001). P53 expression was significantly higher in H. pylori gastritis (p= 0.001), in cagA strain gastritis (p= 0.001), in the presence of atrophic gastritis (p <0.001), and in the presence of intestinal metaplasia (p <0.001). Conclusions The rates of gastric glandular atrophy, intestinal metaplasia, and epithelial proliferation increase in the presence of H. pylori infection, and are further increased when H. pylori is of cag A strain.The rate of apoptosis decreases when precancerous lesions (gastric atrophy and intestinal metaplasia) are present. P53 protein is detectable even before gastric metaplastic/dysplastic change occurs. CagA-positive H. pylori is associated with greater p53 expression. If the p53 protein detected is a consequence of mutation, this would help to explain why cagA-positive H. pylori gastritis is associated with decreased apoptosis.