Risedronate sodium is a pyridinyl bisphosphonate effective for treatment and prevention of postmenopausal and glucocorticoid-induced osteoporosis. Some bisphosphonates have been associated with upper gastrointestinal (GI) tract adverse effects. The objective of this study was to determine the frequency of upper GI tract adverse events associated with risedronate, especially among high-risk patients. The GI tract adverse events reported during 9 multicenter, randomized, double-blind, placebo-controlled studies of risedronate conducted from November 1993 to April 1998 were pooled and evaluated. The evaluation included 10,068 men and women who received placebo (n=5048) or 5 mg of risedronate sodium (n=5020) for up to 3 years (intent-to-treat population). Studies incorporated a comprehensive, prospective evaluation of GI tract adverse events. Adverse event information was collected every 3 months. The treatment groups were similar with respect to baseline GI tract disease and use of concomitant treatments during the studies. At study entry, 61.0% of patients had a history of GI tract disease and 38.7% had active GI tract disease; 20.5% used antisecretory drugs during the studies. Sixty-three percent used aspirin and/or nonsteroidal anti-inflammatory drugs (NSAIDs) during the studies. Upper GI tract adverse events were reported by 29.6% of patients in the placebo group compared with 29.8% in the risedronate group. The risk of experiencing such an event in the risedronate group was 1.01 (95% confidence interval, 0.94-1.09) relative to the placebo group (P=.77). The rate of upper GI tract adverse events per 100 patient-years was 19.2 in the placebo group compared with 20.0 in the risedronate group (P=.30). Risedronate-treated patients with active heartburn, esophagitis, other esophageal disorders, or peptic ulcer disease at study entry did not experience worsening of their underlying conditions or an increased frequency of upper GI tract adverse events overall. Concomitant use of NSAIDs, requirement for gastric antisecretory drugs, or the presence of active GI tract disease did not result in a higher frequency of upper GI tract adverse events in the risedronate-treated patients compared with controls. Endoscopy, performed in 349 patients, demonstrated no statistically significant differences across treatment groups. The results of this extensive evaluation indicate that daily treatment with 5 mg of risedronate sodium is not associated with an increased frequency of adverse GI tract effects, even among patients at high risk for these events.