Microscopic colitis encompassing collagenous colitis, lymphocytic colitis and non-specific microscopic colitis, is a common cause of chronic watery diarrhoea in middle-aged and elderly people, especially women. Its etiology is unknown – infectious, immunologic, hormonal and genetic factors are suspected. An association between use of nonsteroidal anti-inflammatory drugs or proton pump inhibitors and microscopic colitis has been noticed. The disease is more common than it was thought to be. The long-term prognosis is good. Colonoscopy shows normal colonic mucosa, but microscopic diagnostic changes are found in biopsies from both the right and left sides of the colon. In the treatment antidiarrheals with especially bismuth subsalicylate, aminosalicylates, immunosupressive agents and corticosteroids (budesonide) are proposed.