Male and female germ cells vary in their sensitivity to the mutagenic effects of chemotherapy and radiotherapy, depending on their stage of maturation and the agent used. Although sperm DNA damage exists following treatment, no increase in genetic defects or congenital malformations was detected among children conceived to parents who have previously undergone chemotherapy or radiotherapy. The use of assisted reproductive technologies and micromanipulation techniques might increase this risk; hence caution should be exercised. In female cancer patients, miscarriage and congenital malformations are not increased following chemotherapy. However, when IVF and embryo cryopreservation is practised between or shortly after treatment, possible genetic risks to the growing oocytes exist, and hence the babies should be screened. During pregnancy, the potential teratogenic effects of chemotherapy influence the choice and timing of therapy. Termination is usually recommended in the first trimester. Second- and third-trimester exposure does not usually increase the teratogenic risk and cognitive development, but it may increase the risk of poor obstetric outcome and fetal myelosuppression. During the first two weeks after fertilization of the embryo, radiation is lethal but not teratogenic. High doses of radiation during pregnancy induce anomalies, impaired growth and mental retardation, and there may be an increased risk of childhood leukaemia and other tumours in the offspring.