A study has been made of the patterns of vomiting in 305 patients undergoing total body irradiation before bone marrow transplantation. Vomiting usually begins after a dose of 2 to 3 Gy has been given to the whole body and may persist for up to 12 hours from the start of treatment. There is a recurrence of vomiting after 2 to 5 days. It is infrequent in patients receiving fractionated radiation with fraction sizes of less than 2 Gy, and appears unrelated to cumulative dose after the threshold has been reached. The incidence of vomiting is reduced by limitation of patient movement during radiation and by sedation adequate to keep the patient asleep. It is increased by anxiety and there is a higher incidence in patients aged 10 to 40 than in those aged less than 10 years. Other drugs administered before total body irradiation may affect the incidence. Vomiting can be prevented completely in 50% of patients.