Background: We aimed to explore the natural history of irritable bowel syndrome (IBS) in Denmark over 3 years by studying development of IBS symptoms and associated factors. Methods: A cohort study was carried out using a web panel representative of the Danish general population 18-50 years. The survey, including a questionnaire based on the Rome III criteria for IBS, was conducted in January 2010, January 2011, and March 2013. Key Results: The prevalence of IBS was 15.4% (920/5986). The incidence was 10.3%, and was three times higher for persons with unspecific gastrointestinal (GI) symptoms compared to asymptomatic persons. Of respondents with IBS symptoms in both 2010 and 2011, 69% (131/191) also reported symptoms of IBS in 2013, which was significantly more compared to respondents with IBS symptoms in 2010 reporting to be asymptomatic or having unspecific GI symptoms in 2011 (20% and 39%, respectively, P<.001). Being diagnosed with IBS predicted fulfilling the criteria for IBS 3 years later (OR: 2.59, 95% CI: 1.11-6.10). Fulfilling criteria for IBS after 1 year also led to a high risk of IBS symptoms 3 years later in asymptomatic persons and persons with unspecific symptoms at baseline. Conclusions & Inferences: The vast majority of persons fulfilling criteria for IBS report GI symptoms after one and 3 years. Fulfilling IBS criteria after 1 year led to a high risk of reporting IBS symptoms after 3 years. In the general population having an IBS diagnosis predicts persistently fulfilling the Rome III criteria for IBS 3 years later.