Objective: Irritable bowel syndrome (IBS) is a very common condition in general practise, affecting 10-20% of the population in the Western world. The clinical picture of diarrhoea-predominant IBS (IBS-D) resembles other chronic diarrhoeic conditions, such as microscopic colitis (MC). It is impossible to separate these by clinical examinations or lab-tests that can be done in general practise. The aim of this study was to detect any missed diagnoses when only using a symptom-based approach for the diagnosis of IBS. Material and methodology: We examined 87 participants diagnosed with IBS by the Rome III criteria. All the participants underwent full clinical examination, lab-tests and colonoscopy including mucosa biopsies for histological examination. Results: The histological analysis revealed four cases of MC in participants who for years had been diagnosed with IBS. We found no biochemical or clinical markers that made it possible to differentiate between IBS and MC. MC was only found in the participants diagnosed with IBS-D. Conclusion: When long-lasting, unresolved diarrhoeic conditions are present in patients over 45-50 years of age, colonoscopy with biopsy should be performed to rule out MC and other pathologies before diagnosing IBS. In younger patients with pronounced watery diarrhoea, one should consider colonoscopy individually if there is no response to IBS-treatment.