Microscopic colitis has 2 main subtypes, collagenous colitis and lymphocytic colitis, that are similar clinically and histologically and are distinguished mainly by the presence or absence of a thickened subepithelial collagen band. Microscopic colitis accounts for approximately 10% of chronic watery diarrhea and may be associated with abdominal pain and mild weight loss. It is typically diagnosed in the sixth to eighth decade of life, and there is a female pre-dominance that is more striking for collagenous colitis than for lymphocytic colitis. Endoscopic and radiographic findings of the colon appear normal. Diagnosis is made by colonic biopsy, which shows an intraepithelial lymphocytosis and a mixed inflammatory infiltrate in the lamina propria. Many potential treatments have been reported, but few have been subjected to controlled treatment trials. A stepwise approach to therapy often leads to satisfactory control of symptoms.