Among the factors leading to effective treatment of malignant lesions are the precise localization of the involved tissue and its accurate exposure to adequate doses of radiation over a period of time. The use of protracted therapy makes imperative some means of insuring that the treated volume, kept as small as clinically feasible, is being irradiated consistently from day to day. The use of rotational technics accentuates the need for accurate localization methods that can be applied to different sites, with allowance for individual anatomic variations. With the Theratron the requirement of consistency of irradiation is most easily achieved by adopting a standard position for all treatments. In our practice, the patient is in the supine position, squarely on his back, on a full-length, 1-inch thick foam-rubber pad, spread on the treatment couch. With hands either across his chest or at his side, he presents the least chance for variation in the degree of extension of the back, in the amount of pelvic inclination, or twisting of the trunk. At the same time, a base line for the localization scheme becomes available: the flat top of the treatment couch. All irradiation prescriptions contain, as one of the co-ordinates of beam direction, the distance from the plane of the couch top to the central ray of the horizontal beam. The light beam of the localizer is, of course, coincident with the radiation beam. A second co-ordinate is the distance, cephalad or caudad, from some prominent landmark such as the angle of Louis, the pubic symphysis, or some other reference point. The third co-ordinate is the distance laterally from the midline. When a patient has been accepted for radiation therapy on the Theratron, the following steps are carried out prior to treatment. Lateral and anteroposterior roentgenograms are taken with the patient in the standard position, a “ruler” radiograph is made, the volume to be treated is identified and located in terms of the three coordinates, the field size is selected, and the dose is prescribed. At the first treatment session, a lateral radiograph is obtained with the cobalt-60 gamma rays. This is done to confirm the accuracy of the localization procedure. In the material to follow, each of these steps with the exception of the dosage calculation will be discussed in detail. Although this scheme of localization was developed around the use of a Hodges (1) pelvimetry table (Fig. 1), this paper will present the method as it can be used with a standard radiographic table for the anteroposterior view, supplemented by a mobile radiographic unit for the lateral view. The use of a simple cassette holder for the lateral view is desirable, as two films must be exposed without altering the position of the horizontal x-ray beam.