To determine whether the white substance can act as an endoscopic marker for flat esophageal mucosal neoplastic lesions.Esophageal mucosal neoplastic lesions are mainly identified using white light endoscopy, because it is cost-effective; however, this method is limited for detecting early esophageal cancer and precancerous lesions, because these are typically flat mucosal neoplastic lesions. In our experience, a white substance surrounds or covers some flat esophageal mucosal lesions that are eventually diagnosed as neoplastic lesions by biopsy pathology.After retrospective analysis of pathological and clinical data of 20,390 patients, we identified 352 patients with flat esophageal mucosal lesions on endoscopic images. Images were re-evaluated by 2 experienced endoscopists and the prevalence of the white substance recorded. Patients were divided into non-neoplastic and neoplastic groups, based on pathology.The white substance was present in 3.5% (5/144) of non-neoplastic and 14.9% (31/208) of neoplastic cases (P < .05). The diagnostic sensitivity and specificity of the white substance for neoplastic lesions diagnosis were 14.9% and 96.5%, respectively. The presence of white substance was more common in males and in those aged 50 to 79 years. It was more commonly observed in the middle third of the esophagus, and its presence did not correlate with sex, age, or lesion location (P > .05).The white substance, which is easily detected by white light endoscopy, may be an endoscopic marker facilitating detection of flat esophageal mucosal neoplastic lesions, irrespective of sex, age, and lesion location.