This communication describes results with a removable aluminum beam flattener for both normal and wedge field isodose distributions. The work was carried out on the commercially available Model A Eldorado therapy unit. The cobalt-60 source was 2.5 cm. in diameter and measurements were made at an SSD of 80 cm. Initially, it was considered that flattening the beam would be most useful for small fields, up to 10 × 10, and particularly in beam-directed therapy technics. Since the beam fiattener would lower the output, it was decided to make it easily removable for cases where increased output would be advantageous in shortening treatment times. Further, to maintain the skin-sparing effect of cobalt radiation, the filter was placed at a minimum distance of about 20 cm. from the skin surface. Method Beam-flattening filters were made to cover a range of rectangular and square fields up to 100 sq. cm., the dimensions of which are tabulated in Figure 1. The flatteners were fashioned from a solid block but could be...