A significant proportion of patients with osteosarcoma may belong to Li-Fraumeni cancer families. We studied the pedigrees of 17 index patients with osteosarcoma, recording malignant disease and cause of death for first- and second-degree relatives. There were seven cancers and five cancer deaths per 2151.5 person-years in first-degree relatives of osteosarcoma patients under the age of 50 years, a significantly greater incidence than in an age- and sex-matched population group (p < 0.001). This excess of malignancy was largely due to two families which fulfilled the criteria for the Li-Fraumeni cancer family syndrome. Both of these families were shown to have the genetic alterations in the p53 gene which have been implicated in this syndrome. Our study suggests that orthopaedic surgeons seeing new cases of osteosarcoma should arrange screening for familial malignancy.