Familial incidence of Paget's disease and secondary osteogenic sarcoma. A report of three cases from a single family. This report describes a family of three siblings with a long history of polyostotic Paget's disease. Because of the severe osseous involvement with Paget's disease, all three patients had problems that required orthopedic surgery. Subsequent to these surgeries, two patients developed osteogenic sarcoma at sites unrelated to their past procedures, one in the sacrum and one in the calvarium. Both patients died shortly after their diagnosis because of the aggressive spread of the tumor. Although the etiology of Paget's disease and its complication of osteogenic sarcoma still remain to be clarified, this and other case reports suggest a possible environmental or hereditary contribution to developing osteogenic sarcoma in Paget's disease. Patients with a familial clustering of polyostotic Paget's disease may benefit from more thorough screening tests to detect malignant transformation. 