Helicobacter pylori is a gram-negative, microaerophilic bacterium that colonizes human gastric mucosa and affects approximately 50% of the whole world population. It has put the blame on gastric ulcer, duodenal ulcer, chronic atrophic gastritis, mucosa-associated lymphoid tissue lymphoma and stomach adenocarcinoma, as the etiological agent. The cagA (cytotoxin-associated gene A) gene which is one of the most important virulence factors of H.pylori, encodes a 120-145 kDa protein called CagA antigen that may cause cell transformation. The prevalence of cagA positive H.pylori infections varies according to geographical area and age of the patients. Recent studies have suggested that cagA positive H.pylori strains play a role in the development of gastric carcinoma. The aim of this study was to evaluate the prevalence of cagA positive H.pylori isolates in adult and pediatric patient groups in Hacettepe University, Faculty of Medicine. The study was performed on 198 H.pylori stocked strains which have been isolated between 1997-2003 period from biopsy specimens of 107 adult and 91 pediatric patients with gastrointestinal pathology. Chromosomal DNA was extracted by the cetyltrimethyl-ammonium bromide (CTAB) method, and a 348 bp region of the cagA gene was amplified by an "in-house" polymerase chain reaction (PCR) using F1 and B1 primers (Gene Bank number: L11714 position 1231 and 1578R). The evaluation of PCR products revealed that 58.6% (116/198) of the isolates were cagA positive. The rates of cagA positive H.pylori among the adult and pediatric isolates were 62% and 55%, respectively. The present study demonstrates the prevalence of cagA in clinical isolates of H.pylori in our university hospital, and our data was found concordant with the results of studies reported from developed countries.