Osteosarcoma or chondrosarcoma developed as a second malignant neoplasm (SMN) in 40 of 188 individuals with SMN whose first neoplasm occurred in childhood. A genetic susceptibility to cancer was found in 23; the SMN developed in an irradiated field in 32; both factors wer present in 16; neither in one. When a genetic predisposition was present, radiation shortened the interval to SMN. The intervals between tumors and the age at which the bone sarcomas developed in relation to genetic disease and therapy were analyzed by a two-mutation hypothesis. Studies of SMN in childhood permit us to make observation about the role of genetic factors and environmental mutagens in cancer etiology.