Our aims were to investigate the prevalence and subtype distribution of irritable bowel syndrome (IBS) according to the Rome III criteria in Denmark, to describe the difference in symptom reporting between those with gastrointestinal (GI) symptoms not fulfilling Rome III for IBS compared to those classified as IBS, and furthermore to describe the proportion of consulters and formally diagnosed subjects. A web-based survey was carried out in January 2010. Questionnaires were emailed to a web panel (n = 19,567) representative of the general Danish population aged 18-50 years on gender, age, and geography. IBS and subtypes were estimated by the Rome III criteria. Of 6112 responders, 979 (16%) fulfilled the Rome III criteria for IBS and had no organic diagnosis likely to explain their symptoms. Subtypes were: mixed IBS 36%, IBS with diarrhea 33%, IBS with constipation 18%, and unsubtyped IBS 11%. Those with GI symptoms, not fulfilling Rome III for IBS, had symptoms very similar to those classified as IBS, but symptoms were less frequent and of shorter duration. Of IBS subjects, 180/978 (18.4%) had consulted a doctor for GI symptoms within the past 3 months, but only 7.9% were diagnosed with IBS. Symptoms compatible with IBS according to Rome III are highly prevalent in Denmark. A high proportion of sufferers are undiagnosed.