Reports from countries with a high prevalence ofHelicobacter pylori (H pylori) infection do not show aproportionately high prevalence of duodenal ulceration,suggesting the possibility that H pylori cannot be aprimary cause of duodenal ulceration.It has beenmooted that this discrepancy might be explainedby variations in the prevalence of virulence factorsin different populations.The aim of this paper is todetermine whether the published literature gives supportto this possibility.The relevant literature was reviewedand analyzed separately for countries with a high andlow prevalence of H pylori infection and virulencefactors.Although virulent strains of H pylori weresignificantly more often present in patients with duodenalulcer than without the disease in countries with a lowprevalence of H pylori infection in the population,therewas no difference in the prevalence of virulence factorsbetween duodenal ulcer,non-ulcer dyspepsia or normalsubjects in many countries,where the prevalence ofboth Hpylori infection and of virulence factors was high.In these countries,the presence of virulence factorswas not predictive the clinical outcome.To explain theassociation between virulence factors and duodenalulcer in countries where H pylori prevalence is low,only two papers were found that give little support tothe usual model proposed,namely that organisms withthe virulence factors are more likely than those withoutthem to initiate a duodenal ulcer.We offer an alternativehypothesis that suggests virulence factors are more likelyto interfere with the healing of a previously producedulcer.The presence of virulence factors only correlateswith the prevalence of duodenal ulcer in countrieswhere the prevalence of H pylori is low.There is verylittle evidence that virulence factors initiate duodenalulceration,but they may be related to failure of the ulcerto heal. Reports from countries with a high prevalence ofHelicobacter pylori (H pylori) infection do not show aproportionately high prevalence of duodenal ulceration, suggesting the possibility that H pylori can not be aprimary cause of duodenal ulceration. It has beenmooted that this discrepancy might be explained by variations in the prevalence of virulence factorsin different populations. The aim of this paper is todetermine whether the published literature gives supportto this may. The relevant literature was reviewed and analyzed for for countries with a high and low prevalence of H pylori infection and virulence factors. Although virulent strains of H pylori weresignificantly more often present in patients with duodenal ulcer than without the disease in countries with a lowprevalence of H pylori infection in the population, therewas no difference in the prevalence of virulence factorsbetween duodenal ulcer, non-ulcer dyspepsia or normalsubjects in many countries, where the prevalence ofb oth Hpylori infection and of virulence factors was high.In these countries, the presence of virulence factors was not predictive the clinical outcome. To explain theassociation between virulence factors and duodenalulcer in countries where H pylori prevalence is low, only two papers were found that give little support tothe usual model proposed, that that organisms with the virulence factors are more likely than those withoutthem to ainthropothesis that suggests virulence factors are more likely to interfere with the healing of a previously produced ulcer. The presence of virulence factors only correlateswith the prevalence of duodenal ulcer in countries where the prevalence of H pylori is low. There is verylittle evidence that virulence factors, but they may be related to failure of the ulcerto heal.