Malignant disease requiring radiation therapy during pregnancy presents an enormous challenge for the clinician. The optimal radiotherapeutic management of the patient and the optimal management of the pregnancy involve directly opposing demands. Ionizing radiation should be avoided during pregnancy whenever possible. Doses in excess of 0.1 Gy (10 rad) delivered during gestation have been associated with various detrimental effects, and therapeutic abortion has been recommended. If radiation is unavoidable, such as in the treatment of some gynecologic tumors, lymphomatous diseases, or other advanced solid tumors, it must be performed with extreme caution and maximal effort to reduce the dose to the fetus by special shielding techniques. Decisions regarding the use of radiation therapy during pregnancy, the delay of therapy, or pregnancy termination should be made by a multidisciplinary team and be guided by the prognosis of the disease, the stage of gestation, the risk to the fetus from the expected fetal radiation dose, and the patient's ethical and religious beliefs.