Four families were identified in which a pair of children had soft-tissue sarcomas: three sets of sibs and one set of cousins. One parent of each affected child developed cancer; carcinoma of the breast occurred in three mothers under 30 years of age. Other young adults in these families had a high frequency of cancer, with no evidence of underlying genetic disorders known to carry a high risk of neoplasia. The increased familial susceptibility to cancer was manifested not only by the large number of members affected but by a seeming excess of multiple primary neoplasms. These findings suggest a new "familial" syndrome of neoplastic diseases in which heredity or oncogenic agents, or both, may have a causal role.