Helicobacter pylori appears to be a species with strain diversity, according to studies carried out using various molecular biological techniques. Differences in pathogenicity caused by the strain diversity ofH. pylori were therefore investigated. Two hundred and thirty strains were divided into four types by polymerase chain reaction-restriction fragment length polymorphism, and their relationship to endoscopic diagnosis, ammonia concentration of gastric juice, and urease activity were assessed. With regard to incidence by type, patients infected with type 1, type 2, and type 3 exhibited the highest incidences of gastric ulcer, duodenal ulcer, and gastritis, respectively. The urease activity of type 2 was significantly lower than that of types 1 and 3 (P<0.05). These observations suggest differences in pathogenicity due to the strain diversity ofH. pylori. However, the diversity of diseases related toH. pylori is also presumed to be caused by both the diversity ofH. pylori strains and differences in the host immunological reaction. Future studies should be directed toward clarifying the entire pathogenic mechanism ofH. pylori infection.