Microscopic colitis, encompassing collagenous and lymphocytic colitis, is a fairly common cause of chronic watery diarrhoea, especially in elderly women. In recent epidemiological studies the annual incidence of each disorder was 4-6/100.000 inhabitants. The aetiology is unknown. The main clinical symptoms are watery diarrhoea, weight loss and abdominal pain. Laboratory analyses are nondiagnostic, and the diagnoses rely on histopathological examination of colonic mucosal biopsies. There is an association to autoimmune diseases such as thyroid disorders, diabetes mellitus, celiac disease and arthritis. Budesonide is the best-documented treatment of collagenous colitis. It is superior to placebo in short-term therapy, but the long-term efficacy is not well studied. The evidence for other therapeutic alternatives such as loperamide, cholestyramine, bismuth subsalicylate, or 5-aminosalicylates is weak. In unresponsive severe disease azathioprine or methotrexate may be tried. There are at present no controlled data on the treatment of lymphocytic colitis. The long-term prognosis of microscopic colitis is good, serious complications are rare and there is no increased mortality.