Background: The aim of this study was to identify endoscopic predictors for undifferentiated histology prior to endoscopic resection. Methods: Between July 2005 and July 2014, 1,641 early gastric cancers (EGCs) from 1,615 patients were treated by endoscopic submucosal dissection (ESD) at Seoul National University Hospital which were initially identifi ed as adenoma, differentiated EGC, or atypical gland by EFB. The lesions were divided into two groups based on the histological result of ESD: differentiated histology (DH group) and undifferentiated histology (UDH group). The discordant rate, clinicopathologic characteristics and endoscopic factors were analyzed. Results: Of the 1,641 EGCs, 1,556 (94.8%) were diagnosed as DH group and 85 (5.2%) as UDH group after ESD. In UDH group, the mean age was signifi cantly lower (P = 0.004), and female patients were more prevalent (P < 0.001) than DH group. Larger endoscopic size, depressed morphology, ulcer, nodularity, whitish discoloration and absence of erosion were signifi cant predictors for UDH in the univariate analysis. In the multivariate analysis, lesion size > 10 mm (odds ratio [OR] 1.84, 95% confi dence interval [CI] 1.14 - 2.97, P = 0.013), depressed morphology (OR 2.88, 95% CI 1.59 - 5.21, P < 0.001), nodularity (OR 2.89, 95% CI 1.63 - 5.12, P < 0.001), and whitish discoloration (OR 21.00, 95% CI 7.53 - 58.53, P < 0.001) were independent predictors for UDH. Conclusions: Endoscopic size > 10 mm, depressed morphology, surface nodularity or whitish discoloration were the predictors for UDH. Meticulous attention should be paid to the lesions with these endoscopic predictors for the risk of UDH.