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Background: Clinical presentation in microscopic colitis (MC) is similar in many cases to that of diarrhea-predominent irritable bowel syndrome (IBS-D). The proper differential diagnosis requires total colonoscopy with multiple biopsies from normal-appearing mucosa and a detailed histopathological exam. Specific treatment may improve symptomatology. Aim: To evaluate the prevalence of MC in patients with an initial diagnosis of IBS-D, to analyse demographic and clinical features of MC patients and to assess the efficacy of specific treatment. Material and methods: Our retrospective study analyzed patients diagnosed with microscopic colitis in clinic during a three-year period. Diagnosis was established on histological exams of the samples obtained during colonoscopy in patients previously thought to have IBS-D. We evaluated clinical manifestations, time lapsed from their onset to definitive diagnosis, the association of MC with autoimmune diseases or with prior medication and the efficacy of treatment with budesonide or mesalazine. Results: From 247 patients considered to have IBS-D, 15 patients (6.07%) had actually MC (13 lymphocytic colitis and 2 collagenous colitis). MC was associated with nonsteroidal antiinflammatory drugs (3 patients), Lansoprazole (2 patients) and autoimmune diseases (6 patients). Watery, non-bloody diarrhea was present in all patients with MC. Other frequent complaints were nocturnal diarrhea (11 patients), abdominal pain (8 patients), abdominal bloating and flatulence (8 patients) and slight weight loss (6 patients). The diagnostic samples were obtained from the right colon in 6 cases and from rectosigmoid or transverse colon in 9 patients. Treatment was initial symptomatic in all patients, but there were 5 patients that required mesalazine and/or Budesonide, with favourable outcome. Conclusions: All the patients thought to have diarrhea-irritable bowel syndrome should be evaluated for microscopic colitis. Symptomatology is almost superimposable, but a few distinct features can be noticed. The proper and early diagnosis and the specific treatment may lead to significant clinical improvement in some difficult cases of the so-called "irritable bowel syndrome". [ABSTRACT FROM AUTHOR]