The severity of nausea and vomiting in patients undergoing radiotherapy is lower than that associated with chemotherapy regimens, but its duration may be considerably longer. Total body irradiation and irradiation of the upper part of the abdomen or whole abdomen are considered the most emetogenic regimens in radiotherapy. Instead, the emetogenic potential is considered moderate in radiotherapy of the thorax, pelvis and lower half-body irradiation, and low in radiotherapy of head and neck, extremities, brain and skin. In contrast to the very extensive literature on the prevention of chemotherapy-induced emesis, relatively few studies have been published on patients submitted to radiotherapy. Metoclopramide, prochlorperazine and cannabinoids offer only limited symptom control in patients undergoing radiotherapy of moderate to severe emetogenic potential. Double-blind randomized studies showed the superior antiemetic efficacy of the 5-HT3 antagonists with respect to placebo and to the older antiemetic drugs in patients submitted to single dose or fractionated doses of radiotherapy to the upper abdomen, to lower hemibody radiotherapy and to total body irradiation. In all these cases the 5-HT3 antagonists should be considered the antiemetic treatment of choice and should be administered as prophylactic agents. The optimal duration of antiemetic therapy with the 5-HT3 antagonists is unknown. Whether corticosteroids added to the 5-HT3 antagonists will increase their antiemetic efficacy, as in chemotherapy-treated patients, remains to be demonstrated in double-blind controlled trials. Patients submitted to radiotherapy of low emetogenic risk do not require any antiemetic prophylaxis. In this case a rescue antiemetic treatment can be administered if patients present vomiting or moderate to severe nausea.