If the rays we use to destroy cancer cells, or to inhibit their growth, were harmless to all other cells, the solution of the problem of how to cure cancer of the cervix would be easy. We could deliver to every part of the pelvis a lethal cancer dose of roentgen rays, which is now considered to be about 6,000 r. But x-rays are not innocuous, and if we tried to administer, in a short time, such an enormous dose to such a large area, few of our patients would recover. We must compromise between the ideal and the practical. In this country, it is now pretty generally agreed that the best plan of attack on cervical cancer is to administer to the primary tumor, in and around the cervix, a destructive dose, or as nearly a destructive dose as we dare to give. This is supplemented by a smaller dose to the entire pelvis, which we believe, or at least hope, is large enough to inhibit the growth of cancer cells. The most commonly employed method of destroying the primary tumor is to use tubes of heavily filtered radium in colpostats in the lateral vaginal fornices, in combination with an intra-uterine tandem. Although gamma rays have great penetrative power, radium must be placed so close to the tissues to be treated that its action is uneven. With the best possible technic, not more than 30 per cent of the surface dose reaches a depth of 3 cm. (1). This means that, if we deliver a destructive dose to the third centimeter of tissue, the first centimeter receives more than three such doses. The Chaoul technic is also used to deliver a destructive dose of roentgen rays to the cervix. The tube itself is inserted into the vagina and, with good technic (100 kv. and 5 cm. A.S.D.), the distribution of the radiation is about the same as with good radium technic, but no better. About 28 per cent of the surface dose reaches a depth of 3 cm. (2). The transvaginal method of direct roentgenization of the cervix, introduced by Merritt (3, 4), in what I believe to be the most valuable contribution to radiotherapy since the adoption of the international unit, is by far the most efficient method of destroying the primary tumor. The principal objection to it is the difficulty of exposing a field large enough to include the entire lesion and the lateral fornices. Incidentally, this is a good place to say that the treatment of cervical cancer through radio-opaque cylindrical specula, exposing a single field smaller than the lesion, is hopelessly inadequate and should be abandoned. Merritt exposes a large field by using a speculum transparent to the rays, allowing the expanding beam to pass through a part of the vagina. Wasson and Bouslog (5) do it by using multiple overlapping fields, and we do it by retracting the vaginal walls (6, 7, 8). In our modification of the transvaginal method of irradiation, the following physical factors are employed: kv. 200; ma. 20; A.S.D. 25 cm.; effective filter, Cu 0.75 mm.; hardness expressed as half-value layer in copper, 0.9 mm.