One hundred fifty-nine 3-year survivors of childhood soft tissue sarcoma and their relatives were surveyed to determine the frequency of second malignant neoplasms (SMNs) in patients and cancer in their relatives. The cancer experience of the patients, their offspring, siblings, parents, parental siblings, and grandparents was compared to that expected of the general population based on age-, sex- and calendar year-specific rates from the Connecticut Tumor Registry. A significant excess of SMNs was observed in the patients (observed expected = 8:0.38). Among 758 first-degree relatives, a significant cancer excess was observed (34:20.68), attributable largely to cancer of soft tissue and bone (6:0.44) and breast (9:3.39) and to cancers occurring before age 35 years (12:4.14). Overall, a significantly lower than expected cancer incidence was confirmed in the 1,693 second-degree relatives (142:178). To identify patient characteristics associated with higher than expected familial cancer risk, kindreds were partitioned by patient age at diagnosis tumor type, tumor site SMN and other factors. A highly significant cancer excess was observed in the relatives of SMN patients (26:12.78). The tumor types occurring in excess in close relatives were also observed as SMNs in the patients. The findings confirm an association among childhood soft tissue sarcoma and cancers of the breast, bone, joint, or soft tissue as SMN in patients and in close relatives and suggest that the risk of a second tumor is associated with a familial predisposition to cancer.