Lansoprazole is a potent proton pump inhibitor that has been well tolerated with minimal serious adverse events. One of the most commonly reported side effects is diarrhea in 3-8% of study patients. During 1997, approximately 850 veterans at our institution had their proton pump inhibitor converted from omeprazole to lansoprazole because of a formulary change. A number of patients subsequently developed chronic watery diarrhea. While evaluating six of these patients, we discovered microscopic colitis that resolved with discontinuation of lansoprazole. The diarrhea was described as three to 10 loose, nonbloody bowel movements per day with some abdominal cramping. Colonoscopy in five patients and flexible sigmoidoscopy in one patient revealed normal colonic mucosa, but random biopsies all supported microscopic colitis (five cases of lymphocytic colitis and one case of collagenous colitis). Complete symptom resolution occurred in all patients within 4 to 10 days of discontinuing lansoprazole. In all patients, follow-up biopsies demonstrated normalization of the colonic histology. This is the first published case series of patients with microscopic colitis that correlated clinically and histologically with the initiation and discontinuation of lansoprazole.