An osteosarcoma developed near the right lesser trochanter of a 55-year-old woman. The neoplasm fulfilled the generally accepted criteria for a Thorotrast related malignancy. Strengthening this relationship was the occurrence of the tumor in an unusual location and uncommon age group. Thorium was confirmed both in the tumor and in bone marrow histiocytes by its characteristic x-ray spectrum. Including this case only twelve Thorotrast-associated neoplasms of bone have been reported. The mean latency period after Thorotrast administration was 26 years. Regression analysis revealed that latency period and Thorotrast dose are inversely related. All tumors were reported to be sarcomas. Six osteosarcomas, one fibrosarcoma, one chondrosarcoma, and one undifferentiated sarcoma were proven histologically. All patients, for whom follow-up was given, had died of the tumor. The reasons behind the low frequency of reported Thorotrast-associated bone malignancies may be the low concentration of 232Th and radioactive daughters in bone, long latency period or the general lack of knowledge concerning the possible relationship between Thorotrast and bone tumors. Statistical association between Thorotrast injection and bone sarcoma is shown. The criteria for the diagnosis of Thorotrastosis are discussed as well as the long term deleterious effects. The distribution of Thorotrast within the body is discussed as well as the associated radiographic changes.