Cancer in the families of children with soft tissue sarcoma. The cancer experience among 754 first-degree relatives (mothers, fathers, and siblings) of a population-based series of 177 children with soft tissue sarcoma is reported. The current study represents an extension of our earlier work in which the authors found an excess of breast cancer in the mothers of 143 of these children. There were 40 cancers among all first-degree relatives, compared with 24.82 expected (relative risk [RR] 1.61, P = 0.006). There was no excess in fathers, but an excess of borderline significance was seen in mothers (RR 1.67, P = 0.0545), and a significant excess in siblings (RR 4.55, P = 0.0002), mainly due to carcinoma of the breast and pediatric tumors. Results of a step forward Cox multivariate analysis identified three variables in the index child which were independently associated with high cancer risk in relatives, as follows: age younger than 24 months at diagnosis; histologic type, embryonal rhabdomyosarcoma or other and unspecified soft tissue sarcoma; and male sex. It was possible, therefore, to identify a subgroup of children whose relatives are at high risk of early onset cancer (RR in this group 10.14). The pattern of cancers is consistent with the Li-Fraumeni syndrome. The authors conclude that a marked proportion of childhood soft tissue sarcoma has a genetic basis. 