Collagenous colitis (CC) is one of the causes of undefined watery diarrhea, which is histologically accompanied by thickening of the subepithelial collagen layer. CC associated with lansoprazole normally occurs within several weeks after initial administration, but no case presenting after long-term administration of lansoprazole has yet been reported. A 77-year-old male with 6-year history of administration of lansoprazole complained of watery diarrhea and weight loss. Colonoscopy revealed disappearance of vascular networks and red spots in the sigmoid colon. Biopsy specimen showed erosion and collagen bands thickened, so the patient was diagnosed as CC. After lansoprazole discontinuation, the watery diarrhea disappeared and histological abnormalities improved.
