Background: Meckel's diverticulum (MD) is a remnant of the omphalomesenteric duct. Although the majority of MD are asymptomatic, it can present with severe hematochezia. Hematochezia is generally considered to result from a peptic ulcer caused by ectopic gastric mucosa in MD. However, this hypothesis has not been proved. Methods: 10 cases of surgically resected MD initially presenting with severe hematochezia were histologically examined. Results: Ectopic gastric mucosa was present in 9 cases, two of which also contained ectopic pancreas. No ectopic tissue was found in one case, which shows that bleeding can occur in MD without ectopic gastric mucosa. In addition, a rupture of aberrant submucosal arterioles through the overlying mucosa, a vascular abnormality called Dieulafoy's lesion, was detected in all the 10 cases. Conclusion: This study suggests that the actual cause of massive bleeding in MD is not a peptic ulcer, but Dieulafoy's lesion.