The presence of helicobacter pylori in the stomach in patients with clinical signs of gastroduodenal disease and its possible relationship with several clinical, endoscopic and anatomopathologic characteristics of the illness were prospectively evaluated in 359 patients. H. pylori was isolated from 388 samples in 216 patients (60% of colonization). In patients with endoscopic and, or, anatomopathologic evidence of gastroduodenal alterations the microorganism was isolated in 67% of the instances whereas it was observed in only 7% of the patients free of such alterations (p less than 0.001). The prevalence was 100% in cases of bulboduodenitis, 77% in patients with peptic ulcer, 66% in cases of gastritis, and 20% in neoplastic patients. The colonization index for duodenal ulcer was 80% and that of gastric ulcer 70%. Among the cases with gastritis the highest index corresponde to the superficial chronic gastritis (82%). We failed to observe clinical signs that allowed to envisage the presence or absence of H. pylori in the gastric mucosa.