Oncogenic osteomalacia is a rare condition characterized by the development of pain and fractures in a patient with specific laboratory abnormalities consisting of hypophosphatemia, hyperphosphaturia, and decreased 1,25-OH vitamin D levels. The clinical scenario is completed by the association of this osteomalacic state with the finding of a neoplastic process in the afflicted patient. The authors report a patient in whom the diagnosis of oncogenic osteomalacia was established and treatment begun despite the fact that the associated tumor (benign undifferentiated tumor of meschymal origin) escaped detection for many months. Following discovery of the tumor and identification by magnetic resonance imaging, the patient was cured by surgical resection.