The medical records of 973 previously untreated patients diagnosed between January 1960 and December 31, 1978 with childhood cancer were reviewed. Siblings in 13 families were diagnosed with cancer 9/12 to 15 years after the diagnosis of cancer in the index sibling. Previously unreported association of acute lymphoblastic leukemia with Hodgkin's disease, neuroblastoma with malignant hemangiopericytoma, non-Hodgkin's lymphoma with malignant melanoma, Wilms' tumor with non-Hodgkin's lymphoma, Hodgkin's disease with malignant teratoma of the testis and craniopharyngioma with acute myeloblastic leukemia were identified. Two families appeared to transmit a predisposition to childhood tumors. The data from these families extend previous observations regarding multiple cases of cancer in sibships.