The "subperiosteal bone cyst", first described by Brooks and Lehman, is an unusual manifestation of multiple neurofibromatosis. It consists of a neurofibroma which probaly arises from a periosteal nerve, elevates the periosteum from the cortex, and stimulates the rapid formation of a lamella of new bone which partially encapsulates the tumor. Neurofibromatous tissue can also be identified in the periosteal layer on the convex Side of the newly formed bone, and direct continuation of neurofibromatous tissue can be demonstrated between the tissue on the convex side and on the concave side of the new bone lamellae. There is no invasion of the cortex with this lesion; and, with its rapid development, it may pose a problem in diagnosis which must be resolved by exploration and removal. Disturbances of bone growth in this case do not seem to be dependent on direct neurofibromatous involvement of bone, and such aberrations of bone growth may be interpreted as evidence of a congenital mesenchymal defect.