Lynch syndrome and inflammatory bowel diseases (IBD) are associated with an increased risk of colorectal cancer (CRC). However, it is not clear whether the risk of CRC is even higher for patients with a combination of Lynch syndrome and IBD. We investigated the risk for CRC in this subgroup by establishing a Lynch syndrome cohort from the Radboud University Medical Center (Nijmegen, The Netherlands) and the Academic Medical Center (Amsterdam, The Netherlands). Patients with heterozygous germline mutations in MLH1, MSH2 (and EPCAM deletion-mediated MSH2 methylation), MSH6, or PMS2 who were tested and/or treated from 1998 through 2014 were included. Patients who developed IBD were identified by linkage of this cohort to the Dutch nationwide Pathology Registry (PALGA). Subsequently, we compared the risk of CRC between Lynch syndrome patients with IBD and without IBD. Of 1046 patients with Lynch syndrome, 15 developed IBD (1.4%). Patients with Lynch syndrome and IBD were significantly younger (median age, 38.0 y) than patients with Lynch syndrome without IBD (median age, 52.0 y; P = .001). Nevertheless, a similar proportion of patients in each group developed CRC: 4 of the 15 patients (26.7%) with Lynch syndrome and IBD compared with 311 of the 1031 patients (30.2%) with Lynch syndrome without IBD. Patients with Lynch syndrome and IBD developed CRC at a younger age (median age, 36.0 y) than patients with Lynch syndrome without IBD (median age, 46.0 y; P = .045). However, the cumulative incidence of CRC was similar between groups (P = .121). All patients with Lynch syndrome and IBD who developed CRC had ulcerative colitis, producing a higher cumulative incidence of CRC for this IBD subgroup (P < .001). In conclusion, patients with Lynch syndrome and IBD develop CRC risk at a younger age than patients without IBD; patients with ulcerative colitis are at especially high risk.