OBJECTIVE To outline the evidence for and against ulcerogenic strains of Helicobacter pylori. Although H. pylori is now well established as a predisposing factor in peptic ulcer disease the question of why only a minority of people infected with H. pylori develop peptic ulceration remains unanswered and is the subject of much controversy. Perhaps the most accepted view to date is the belief that there are ulcerogenic strains of H. pylori which produce a specific cytotoxin responsible for the ulceration. CONCLUSIONS At the present time the case for ulcerogenic strains is not proven and the differences between H. pylori infected peptic ulcer patients and H. pylori infected non-ulcer subjects may relate simply to host factors.