A highly increased risk of myelodysplasia (MDS) and acute myeloid leukemia (AML) has been demonstrated following therapy with alkylating agents. The risk increases with cumulative dose and with the age of the patient. Most cases of MDS and AML following therapy with alkylating agents present chromosome aberrations, primarily loss of whole chromosomes No. 5 and/or No. 7 or various parts of the long arms of these chromosomes. The risk of MDS and AML following high-voltage radiotherapy is much lower. Recently an increased risk of AML has been demonstrated following therapy with the epipodophyllotoxins etoposide and teniposide. These leukemias typically present without preceding MDS and often show balanced aberrations of chromosome bands 11q23 and 21q22.