AIMS: To examine the potential role of the lipolytic enzyme phospholipase A2, produced by Helicobacter pylori in ulcer formation. METHODS: Phospholipase A2 activity in H pylori was compared with that in 10 commonly occurring pathogenic bacteria. Phospholipase A2 activity and its cytotoxic metabolite, lysolecithin, in the basal gastric aspirates of 12 patients infected with H pylori were compared with those in 12 subjects not infected with H pylori. RESULTS: The phospholipase A2 activity in H pylori was substantially higher than that in most of the other bacteria tested, and the activities of phospholipase A2 and lysolecithin in the basal gastric aspirates of those infected with H pylori were significantly higher than the activities found in the basal gastric aspirates of subjects who were not infected. The lysolecithin proportion of total phospholipids in the gastric aspirates was also much higher in the infected than the non-infected group and a weak positive correlation (0.415) was found between phospholipase A2 and lysolecithin in the infected group. CONCLUSIONS: H pylori has clinically important concentrations of phospholipase A2, an enzyme capable of hydrolysing gastric mucosal phospholipids. The high values of phospholipase A2 and lysolecithin in gastric fluid from subjects with H pylori infections supports the notion that phospholipase A2 is involved in the inflammation and mucosal damage associated with peptic ulcer formation.