Acute superior vena caval obstruction is one of the true emergencies encountered in radiotherapy. If untreated, the obstruction not only renders the patient helpless, but heralds death by suffocation. The main aim of any therapeutic program is to obtain rapid relief of this syndrome and avoid its recurrence. The patient with this condition is inevitably hospitalized, and consultants of different disciplines are often sought in determining the ideal management. The clinical literature unfortunately is indecisive as to the best treatment, and conflicting points of view confuse rather than clarify the issue. In view of the moribund status of the patient, there is a tendency for radiation therapy schedules to be conservative, particularly since an untoward reaction, as “radiation edema,” may tip the balance and lead to sudden death. The results of our experimental studies (p. 406) indicated that progressive edema is due to uncontrolled tumor growth rather than to irradiation (“radiation edema”). Cautious slow-dose schedules were unsuccessful, whereas high rapid-dose schedules were not only well tolerated but led to rapid relief in the majority of the animals. In the light of these findings, we have modified our clinical approach to this problem. Our initial observations are offered in this report. Diagnostic Considerations 1. Initial Study: History and physical examination should permit a working diagnosis in the emergency room. The classical physical signs are edema of the face, arms, and chest, congestion of face and conjunctivae, prominent venous collateral circulations, with aggravation of findings on recumbency. A chest film, often demonstrating a mass in the superior mediastinum, completes the first phase of examination. Because of the emergency and progressive nature of this syndrome, radiation therapy is instituted immediately and the diagnostic work-up is begun simultaneously. This is a departure from our usual policy of not accepting patients without pathologic proof of malignancy. All too often a patient becomes moribund while the house staff awaits confirmation of tests and biopsies before referral for radiotherapy. 2. Routine Confirmatory Tests: (a) Infrared photography is simple and effective in detailing collateral circulation over the chest wall and is worth following as a parameter of response to treatment. (b) Venous pressures have received much attention, but simple pressure recordings with a spinal fluid manometer in an arm vein 10 cm. below the anterior chest wall are sufficient to establish elevations, which may vary from 150 to over 350 mm. saline. It may be of interest to test both arms to determine if the innominate vein is also obstructed. Femoral vein pressures have been recorded to determine if the pressure rises with inspiration, in which circumstance the obstruction is below the entrance of the azygos vein; otherwise, it is above.