Currently, transnasal esophagogastroduodenoscopy using an ultrathin endoscope is being widely carried out as a screening test for early gastric cancer. We compared the diagnostic utility of ultrathin esophagogastroduodenoscopy with that of conventional esophagogastroduodenoscopy in detecting 42 lesions of early gastric cancer that had a diameter of <or=20 mm. Only 27 lesions (64%) could be accurately diagnosed using ultrathin esophagogastroduodenoscopy. In nine lesions (22%), we failed to discern whether they were malignant. Six lesions (14%) could not even be detected. We found that the diagnostic utility of ultrathin esophagogastroduodenoscopy was inadequate, especially in the case of lesions that were located in the upper third region of the stomach and variegated lesions. In conclusion, the diagnostic utility of ultrathin esophagogastroduodenoscopy might be lower than that of conventional esophagogastroduodenoscopy in terms of screening for early gastric cancer. The disadvantages of ultrathin esophagogastroduodenoscopy should be taken carefully into consideration while examining lesions.