Increasing numbers of young people are now being cured of certain neoplasms by radiotherapy and chemotherapy. Such people will naturally wish to lead a normal life and possibly to have children. Therefore the question of the effect of radiation and cytotoxic drugs on the reproductive capacity of these patients has become important. The purpose of this report is to review the information available on the effect of radiation on fertility in man. Direct information on radiation effects on human fertility is available from reports on therapeutic exposure and deliberate experimental exposure. Although the total number of cases involved is small, a number of general principles emerge. In males, fractionated irradiation of the testes may be more harmful than acute, at least up to total doses of about 600 cGy (rad). Fractionated doses greater than 35 cGy cause aspermia, the time taken for recovery increasing with dose, and after more than 200 cGy aspermia may be permanent. In females, response varies with age as well as dose. For example, 400 cGy may cause a 30% incidence of sterility in young women, but in women aged above 40 years it results in 100% sterility. However, individuals of either sex show a degree of variation in their response to irradiation.