Background Endoscopic submucosal dissection (ESD) is an effective treatment of early gastric tumors, but submucosal fibrosis can be an obstacle to successful ESD. Objective To examine the association between endoscopic and pathologic factors and submucosal fibrosis in early gastric tumors, and to measure the association between degree of submucosal fibrosis and outcomes of ESD. Design A retrospective study. Setting An academic medical center. Patients From November 2006 to April 2011, 161 patients with 167 early gastric tumors treated by ESD. Intervention ESD. Main Outcome Measurements Endoscopic and pathologic factors related to submucosal fibrosis. Procedure time, en bloc resection rate, and complications according to degree of submucosal fibrosis. Results In univariate analysis, the presence of endoscopic submucosal fibrosis was significantly related to tumor size, location, ulceration, histologic findings, and submucosal invasion. Multivariate analysis for these factors showed that endoscopic submucosal fibrosis was independently associated with lesions in tumor size greater than 30 mm, in the proximal portion of the stomach, and more common in adenocarcinomas than in adenomas. After correction for multiple testing, only the middle of the stomach as a locational risk factor retains statistical significance. Also, the more advanced the endoscopic submucosal fibrosis, the longer the time required for ESD (P < .0001). The severity of endoscopic submucosal fibrosis was associated with a lower en bloc resection rate and with abundant immediate bleeding. Limitations Retrospective, single-center study. Conclusion Submucosal fibrosis of early gastric tumors is closely related to tumor size, location, ulceration, histologic findings, and submucosal invasion. Moreover, the greater the degree of submucosal fibrosis the longer the time taken for the ESD procedure and the higher the frequency of complications such as perforation and immediate bleeding.