When treating Crohn disease (CD) with thiopurines, achievement of an objective response is essential. However, the minimal degree of mucosal improvement required to alter disease outcomes of CD is unknown.To determine the endoscopic responses of thiopurine monotherapy and to determine the minimal degree of mucosal improvement required to alter disease outcomes of CD.One hundred thirty CD patients who had evaluable ileocolonoscopy with evident of mucosal ulceration at baseline were included. The endpoints were endoscopic responses at the 2 follow-up endoscopies performed at 12 months (M12) and 36 month (M36) from the initiation of thiopurines.At M12, mucosal healing (MH) and a positive endoscopic response (PR) were documented in 38% and 46% of patients, respectively. At the second follow-up, merely a further 14% (13/93) of patients on monotherapy had a PR and a total of 46% (43/93) presented with MH. In a Cox regression model, both a PR (P < 0.02) and MH (P < 0.001) at M12 were associated with response at M36 in patients continuing thiopurine treatment. MH at M12 was associated with long-term disease outcomes of CD at M36, with an area under the Receiver Operator Characteristic curve of 0.54 for predicting clinical remission, 0.69 for hsCRP normalization, 0.69 for MH, and 0.74 for PR, respectively. A PR at M12, defined as a decrease in endoscopic activity score by ≥2 points from baseline, yielded similar results.Endoscopy at M12 can help to identify responders to thiopurine monotherapy in active CD. A PR could represent the minimal clinically important improvement in endoscopic disease activity.