National Danish guidelines for the diagnosis and treatment of irritable bowel syndrome (IBS) in adult patients in secondary and tertiary care have been approved by the Danish Society for Gastroenterology and Hepatology. IBS can be a positive diagnosis in patients fulfilling the Rome III criteria for IBS with no alarm signals, a normal physical examination and a normal CRP and hemoglobin. In patients < 40 years with IBS and diarrhea, a normal fecal calprotectin excludes inflammatory bowel disease with a high probability. Patients with IBS and diarrhea should be tested for celiac disease. Endoscopy is not routinely recommended. The therapeutic gain of various treatment modalities is small and most likely overestimated in older studies. However, side effects are usually mild which may justify empirical treatment. The choice of therapy based on IBS subtyping is pragmatic and there are only few trials as guidance. The significance of previous failure with another treatment modality is unclear. There is a lack of long-term treatment trials. The generalizability of the trials is poor, mainly due to selection bias.