The combination of bilateral lytic lesions in the bones of the lower and upper extremities and presenile dementia is characteristic of polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy, also known as Nasu-Hakola disease. The clinical course of this rare and fatal disorder is characterized by pathologic fractures of these often painful lesions, rapid progression of dementia, and death in the fifth decade of life. The radiographic changes may be confused with cystic angiomatosis, focal metastasizing hemangioendothelioma, or Langerhans' cell histiocytosis. We report five patients to illustrate the clinical presentation, radiographic images, psychiatric abnormalities, and new genetic findings. Three of the patients were siblings. A biopsy is not needed to confirm the diagnosis of polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy because of the unique combination of radiographic and neurologic features.