In some patients with extreme phenotypes of inflammatory bowel disease (IBD; in particular in those with infantile or very early onset IBD with <6 years of onset, nonresponsiveness to therapy, or extraintestinal manifestations), Mendelian forms of IBD have been described.1 To identify patients with Mendelian disease-associated IBD in a cohort with age at diagnosis between 7 and 40 years of age, we screened for 59 Mendelian forms of IBD by exome sequencing across 503 patients with IBD with severe disease, as indicated by need for intestinal surgery and/or therapy progression to biologics (Supplementary Material).