Despite so much contributions reported in the literature, the aetiology of the duodenal ulcer remains an enigmatic subject to the medical profession. Findings of Helicobacter pylori seem to have overshadowed the real issue, in that, how a small area of the duodenal mucosa could be inflicted with the acid-pepsin injury has not been questioned? One hundred and sixty-eight consecutive patients, presented with epigastric pain were included in the endoscopic study. The aim of the study was to find out the prevalence and its clinical importance on the sizes of the pyloric aperture in the aetiology of peptic ulcer disease. Demographic data on the sizes of the pyloric aperture were divided into two groups, in that, those up to 3 mm in diameter were included in one and those over the size of 3 mm in another. Among the 168 cases, the gastric ulcer was found in 12 and duodenal ulcer in 27 patients. The sex ratio of men to women was 1.4:1 found in the former and 8:1 in the latter. Among other findings, a knuckle of duodenal mucoa was noticed prolapsing through the large pyloric aperture. It could be postulated that a knuckle of the mucosa that keeps peeping through the pylorus acts as a mucosal plug in empty stomach, like a cork in the acid bottle. The main physiological function is to protect the mucosa from being damaged by the acid-pepsin injury or by the reflux of bile, but the tip of the plug seems to be subjected to such injury. Furthermore, the surface epithelial cells could also be subjected to ischaemic change while prolapsing through the pylorus. This may lead to reduced production of the mucosal gel and bicarbonate secretion, thus exposing the damaged mucosa to acid bath. This supports the concept, how a small area of the stomach or duodenum could be inflicted with ulceration.