Information was obtained on the health status or cause of death in the mothers of a population-based series of 143 children with soft tissue sarcomas. Among these mothers there were 6 cases of breast cancer. All 6 women were pre-menopausal and 2 had bilateral disease. This represents a significant 3-fold excess risk of breast cancer. Malignant disease had occurred in 6 other women whose ages at diagnosis ranged from 33 to 58 years. This was not significantly in excess of expectation. The incidence of cancer among mothers of various sub-groups of children was computed. For breast cancer mothers of: boys, children who were less than the median age at diagnosis, and children who had pelvic tumours had a greater excess risk than the group as a whole. Among those sub-groups of mothers the highest excess risk was 13.5. For other cancers, no sub-group showed an incidence which was significantly above the expected. A high proportion of infiltrating lobular carcinoma was found among the breast cancers, and histological type may indicate familial disease. These findings are consistent with the cancer family syndrome described by Li & Fraumeni in 1969, but the present results suggest that a higher proportion of childhood soft tissue sarcoma than was hitherto suspected may have a genetic aetiology. Further pedigree and laboratory studies may help to identify familial cases at the time of the child's diagnosis.