BACKGROUND & AIMS: Mucosal healing might alter midterm and long-term outcomes of patients with Crohn's disease (CD) and has become an important end point in clinical trials. However, the minimal degree of mucosal improvement (endoscopic response) required to alter midterm outcomes is not known. We aimed to determine the best definition of endoscopic response by evaluating data on the Simple Endoscopic Score for Crohn's Disease (SES-CD) and the Crohn's Disease Endoscopic Index of Severity (CDEIS) from the Study of Biologic and Immunomodulator Naive Patients in Crohn's Disease (SONIC trial). METHODS: We analyzed data from 172 patients who participated in the SONIC trial, were found to have endoscopic lesions at baseline, and underwent a second endoscopic examination at week 26 of treatment with infliximab, azathioprine, or both. Mucosal healing was defined as absence of ulcers. A central reader calculated SES-CD and CDEIS results. Different cutoff values were set for endoscopic response based on the SES-CD or CDEIS. The diagnostic ability of these different cutoff values was evaluat RESULTS: Based on analyses of ROC curves, PLR, and NLR, endoscopic response was defined as a decrease from baseline in SES-CD of at least 50%. At week 26, mucosal healing and endoscopic response were achieved in 48% and 65% of patients, respectively. Mucosal healing at week 26 was associated with CFREM at week 50, with 56% sensitivity, 65% specificity, a PLR of 1.60, and an NLR of 0.67. Endoscopic response at week 26 was associated with CFREM at week 50, with 74% sensitivity, 48% specificity, a CONCLUSIONS: In patients with CD, mucosal healing and endoscopic response (defined as a decrease from baseline in SES-CD or CDEIS of at least 50%) at week 26 of treatment identified those most likely to be in CFREM at week 50. The ability of the proposed endoscopic response cutoff value to predict midterm CFREM should be validated in an independent, prospective cohort. Its correlation with changes in long-term disease progression still needs to be demonstration.