Every effective tumor therapy is, to a certain extent, accompanied by side effects and this applies also to radiotherapy. A large number of side effects occuring during radiotherapeutic treatment is reversible. The chronic radiation injuries remain, in general, asymptomatic. They can be considerably reduced by the use of megavolt machines, a sophisticated computer-assisted treatment technique and with the help of concomitant medication therapy. While earlier, an incipient reaction to radiation appeared on the skin, during the era of high-voltage therapy, the important radioreactions developed deeper in the human body. Besides the reaction of the oral and pharyngeal mucosae, radiation pneumonitis or radiation pneumosclerosis, radiation enteritis and radiation proctitis are important. These are the reactions which the family physician is most frequently confronted with. Possible prophylactic measures and therapy are discussed. The responsibiliy for radiotherapeutic side effects and risks must be assumed jointly by the physicians involved, the patient must be informed about them and efforts made to gain his cooperation.