abstract 171 patients with duodenal ulcer, gastric ulcer, chronic gastritis and gastric cancer were studied. the last 3 were histologically confirmed. 56 cases with duodenal ulcer, 48 with gastric ulcer, 47 with chronic gastritis and 20 with gastric cancer were analyzed. the presence of helicobacter pylori was detected by pcr in 98.2 % of the duodenal ulcers; in 95.8 % of the gastric ulcers; in 95.0 % of the gastric cancers; and in 93.6 % of the chronic gastritis. the cag a genotyping of the strains found proved to be positive in 80.0 % of the duodenal ulcers; in 72.7 % of the chronic gastritis; in 69.6 % of the gastric ulcers; and in 42.1 % of the gastric cancers, for a total prevalence of 70.7 %. both, the ulcers as a whole and the chronic gastritis showed a prevalence of cag a+ strains of helicobacter pylori significantly higher than gastric cancer (p = 0,19).