Osteitis deformans (Paget's disease of bone) may be complicated by osseous cancer. Of 3964 patients with Paget's disease who were seen at the Mayo Clinic between 1927 and 1977, 38 had primary malignant bone tumors. This represents an incidence of sarcomatous change of 0.95%. The bones of the pelvis were most often involved; the humerus and the femur were next in frequency. No region of the skeleton was spared, with the exception of the forearms and hands. Of the 38 tumors, six were fibrosarcomas and 32 were osteosarcomas. Irrespective of the site of involvement, the histopathologic features, or the mode of treatment, the prognosis was extremely poor. The 5-year survival rate was 7.9%, with a mortality rate of 86.8% 36 months after diagnosis. The main cause of death was pulmonary metastasis or local extension of tumor growth. Three patients were long-term survivors, but no clinical or pathologic elements could be identified which differentiated these survivors from the other patients in the study.