Lymphocytic colitis: A type of inflammatory disease of the large intestine (colon). The name derives from the microscopic observations of biopsies of the colon. What is seen in the microscope view of colon tissue is an increased number of inflammatory white blood cells (lymphocytes) among the lining cells of the colon. The elderly are most commonly affected by lymphocytic colitis, and symptoms of the condition typically include a chronic, watery diarrhea without the presence of blood. Abdominal pain and cramping may also occur in people with lymphocytic colitis.

The cause of lymphocytic colitis has not yet been determined, but some doctors believe that an infection or an autoimmune process may be involved in the development of the condition. Since the colon (large intestine) appears normal under colonoscopy, this condition has been referred to as "microscopic colitis" because the characteristic abnormalities are only apparent when biopsies of the colon are examined microscopically.