The most recently recognized and least understood forms of inflammatory bowel disease are two types of idiopathic microscopic colitis-collagenous colitis and lymphocytic colitis. These disorders share many clinical and histologic features, but they have a few notable differences. Whether these are two distinct entities or different presentations of the same disease remains controversial. Bismuth subsalicylate is effective in some patients and treatment is symptomatic and empirical in others. Unlike ulcerative colitis and Crohn's disease, these forms of inflammatory bowel disease rarely require surgery. Ileostomy with or without colectomy is curative for the rare patient with severe symptoms that are refractory to medical measures. Collagenous and lymphocytic colitis are not associated with an increased risk of malignancy.