The Late Effects Study Group (LESG) has collected 368 cases of second malignant neoplasms (SMN) in patients having been previously treated for a first malignancy during childhood. The most frequent first neoplasm is retinoblastoma, followed by Hodgkin's disease. The second neoplasms are essentially sarcomas mostly associated with radiation therapy. Leukemias are also frequent, apparently essentially induced by alkylating agents. The incidence of SMN depends on the primary and on the treatment given, thus it varies according to the studied cohorts. In the most recently analysed material of the LESG, the 20-year incidence was 8%. Children treated for a first neoplasm have an elevated risk of developing a SMN. Genetic predisposition may increase that risk.