Helicobacter pylori (H. pylori) is the major causal agent of gastritis, peptic ulcer and gastric cancer. Several bacterium genes seem to be involved in the pathogenicity mechanism. One of them, the cagA gene, has been extensively studied and characterized. In this article we have carried out a study of characteristics and genetic variability of cagA gene in different geographic areas of the world. At the same time, we have summarized several studies that evaluate possible relation of cagA with gastroduodenal diseases associated by H. pylori infection. In our study we found that the presence of the cagA gene has been confirmed in more than 60% H. pylori strains distributed throughout the world. The prevalence of cagA genotype is of 65.4% in gastritis patients, 84.2% in patients with peptic ulcer and 86.5% in those with gastric cancer. It shows a high genetic variability of cagA associated with gastroduodenal diseases that could serve as a virulence marker in H. pylori infected subjects. However, the high prevalence of H. pylori cagA positive strains in some geographic areas does not confirm the strong association between cagA and virulence of strains as described in other countries. Nowadays, cagA gene is considered as a marker for the presence of cag pathogenicity island (cag-PAI) in H. pylori genoma. This region contains several genes that has been involved with the production of cytokines that results in an increased inflammation of host gastric mucosa, but its function is unknown. Probably, others bacterium factors, such as susceptibility host and environmental cofactors could influence in the risk of developing different gastroduodenal diseases associated with H. pylori infection.