OBJECTIVE Collagenous colitis and lymphocytic colitis (collectively known as microscopic colitis) are characterized by chronic diarrhea, normal endoscopic and radiologic findings, and typical findings on histologic examination of colonic tissue. The purpose of this study was to define the background characteristics of patients with microscopic colitis, as well as to present symptoms, coexistent autoimmune diseases, and a possible association with the use of non-steroidal anti-inflammatory drugs (NSAIDs) and ticlopidine. METHODS A retrospective chart review was carried out on all cases of collagenous colitis and lymphocytic colitis diagnosed at a single center from July 1992 to July 2002. RESULTS Of the 104 patients identified, 66 had collagenous colitis, 35 had lymphocytic colitis, and 3 were diagnosed with both disorders at different times. The mean age of patients was 64 years (26-88 years), with a female:male ratio of 4.8:1. The most common presenting symptoms were diarrhea (95%), weight loss (41%), abdominal pain (40%), fecal urgency (29%), and nocturnal stools (22%). Autoimmune disease was diagnosed in 29% of patients, 35% were using an NSAID, and 2% were using ticlopidine. CONCLUSIONS Collagenous colitis and lymphocytic colitis occur more often in females than in males, at a wide age range, with a mean in the seventh decade. Certain symptoms are characteristic, but are not specific to these disorders. There may be an association with the presence of a coexistent autoimmune disorder and the use of drugs such as NSAIDs.