Purpose- To estimate the incidence of adverse gastrointestinal events in alendronate users.Methods- The computerized pharmacy claims of 12 geographically dispersed United Health Group-affiliated health plans were used to identify 1421 persons who received alendronate prescriptions. The medical claims data of these individuals were searched for subsequent diagnoses of oesophagitis, ulcer of the oesophagus, oesophageal perforation, gastric ulcer, and gastritis/duodenitis. The incidence level was estimated as cumulative incidence and incidence density and their 95% confidence intervals.Results- Thirty-nine persons had a diagnostic code indicating an incident oesophageal or gastric diagnosis of interest, including 22 with oesophagitis, two with oesophageal ulcer, one with gastric ulcer, and 15 with gastritis/duodenitis. Thirteen cases (33.3%) underwent upper endoscopic examination. Five (12.8%) patients were hospitalized. Reflecting alendronate use, 95% of patients were female. The cumulative incidence of oesophageal and gastric events for alendronate users was 3.1% in females, 2.0% in males, and 3.0% overall. The incidence density of a diagnosis of oesophageal or gastric events was 9.0 per 100 woman-years of exposure. There was no significant variation in the cumulative incidence among different age groups.Conclusions- These results suggest the incidence of oesophageal and gastric diagnostic codes is high among alendronate users. Further research is needed to assess the possible association between alendronate and adverse upper gastrointestinal events. Copyright (c) 2000 John Wiley & Sons, Ltd.