Ever Since colpostats were first designed, there has been an attempt to find one that (1) would be easy of application; (2) would maintain a steady position in the f ornices without slipping or rotating; (3) would facilitate or eliminate packing; (4) would allow an increase in the total number of milligram hours by better distribution of the radium source and by the addition of differential screening in the direction of the parametria without exceeding the tolerance of bladder and rectum. Some applicators, such as those of Neary and Blomfield (1, 3, 9), have been designed on the basis of calculations. They are theoretically superior because they are built with considerable differential screening which allows an increase in dose at point B up to 5,000 gamma roentgens without exceeding the tolerance of bladder or rectum. The disadvantages of using expensive metal and large amounts of radium are no longer valid, since uranium and cobalt60 are available. Such applicators, however, have the disadvantage that, being heavy, they rest low in the pelvis, thereby keeping the radium sources at a greater distance from the broad ligament and the pelvic nodes. Recent knowledge as to the distribution of metastatic nodes has further emphasized the importance of increasing the contribution to the hypogastric and external iliac nodes, which are located high and posteriorly on the pelvic wall. For these reasons, extensive clinical research should be done before widespread use of a procedure departing so radically from thoroughly proved technics. Some applicators combine the uterine tandem and the colpostat in one rigid piece (2, 4). These have the advantage of a fixed geometry between the radium sources, making calculations simpler. They have also disadvantages: 1. Separate plastic or rubber uterine tandems can be fitted better to the length of the uterine cavity. This is essential for adequate irradiation of the cavity, which is often involved. Also, according to recent direct measurements, a greater contribution is thus made to the hypogastric, external iliac, and sacral nodes. 2. The vaginal radium sources are located lower in the vagina. This is especially true in cases of exophytic tumors, in cervices greatly enlarged by infiltrating tumor, in unilateral obliteration of the fornices, and in contraction of the vault due either to senility or disease. In the latter instance the uterine tandem will protrude in the vault and the volume of adequate radiation is brought dangerously close to the level of the internal os. 3. It is often desirable to apply the vaginal radium first (infection, impossibility of finding the os until tumor has regressed, exophytic tumors or cervix greatly enlarged by infiltrative carcinoma). 4. These applicators traumatize, to fit their own geometry, structures which are fixed by the tumor or previous inflammatory conditions.