We report three cases of children with moyamoya disease who revealed prominent leptomeningeal enhancement on postcontrast magnetic resonance (MR) images. Two of the children underwent bilateral extracranial-intracranial bypass operations because of ischemic symptoms. During the operations, a profuse pial arterial network was observed, which was considered to be the origin of the enhancement. The ischemic episodes disappeared after the operations, and the follow-up MR images showed marked reduction of the leptomeningeal enhancement. One of the three patients had an extracranial-intracranial bypass operation on one side, but postoperative MR images have not yet been obtained. We named this finding on postcontrast MR images "ivy sign," because it resembles ivy creeping on stones. Ivy sign seems to be a unique, characteristic feature of the pial network observed in moyamoya disease.