Background: Current data on the prevalence of inflammatory bowel diseases (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), within the United States (US) are lacking. We sought to generate population-level estimates of the prevalence of IBD in the US overall and among selected age, sex, racial and ethnic groups. Methods: This retrospective cohort study pooled insurance claims data from Medicaid (government funded for low income), Medicare (government funded for elderly and disabled), and two commercial insurance claims databases (Optum Clinformatics and Anthem). The prevalence of IBD was estimated using data up to 2012 for Medicaid and up to 2017 for all other data. We analyzed all individuals with a minimum of 4 years of available data to allow adequate time to fulfill any of the following case definitions: (i) Â2 diagnoses for IBD with at least 1 by a gastroenterologist or surgeon or (ii) 1 diagnosis of IBD and treatment with an IBD therapy (5ASA, anti-TNF, thiopurine, methotrexate, anti-integrin, anti-IL12/23, JAK inhibitor) and no other indications. Disease subtype was based on the most frequent and recent of CD or UC diagnosis. Patients who could not be categorized as CD or UC were included in overall estimates of IBD. Direct standardization to the 2018 US census was used to generate age-, sex-, and insurance-standardized estimates. Race and ethnicity estimates employed racial categories included in Medicaid, Medicare and Optum Clinformatics data. Results: We analyzed 17,968,322 individuals of whom 125,524 met definition (i) or (ii). The age-, sex- and insurance-standardized pooled prevalence per 100,000 population for IBD was 784.7 (95% confidence interval 780.1 - 789.2), CD 320.8 (317.9 - 323.7), and UC 361.9 (358.9 - 365.0). Extrapolated to the 2020 census estimate of 331,449,281 US population, we estimate that there are approximately 2.6 million Americans diagnosed with IBD, 1.1 million with CD, and 1.2 million with UC. The age-and sex-specific prevalences are summarized in Figure 1. Prevalence in non-Hispanic white Americans was more than 50% higher than in non-Hispanic Black, non-Hispanic Asian or Hispanic Americans (Figure 2). The ratio of UC/CD was 1.1/1 in non-Hispanic white, 1.1/1 in non-Hispanic Black, 1.4/1 in non-Hispanic Asian and 1.5/1 in Hispanic Americans. Conclusions: The prevalence of IBD is close to 1% of the US population and represents a substantial public health burden. The prevalence in white Americans exceeds that of other racial and ethnic groups by more than 50%; better understanding of the determinants of disease and patterns of care across racial and ethnic groups is needed.