The maximum dose rate being delivered to the base of the bladder during intracavitary therapy was assessed in 20 patients by CAT scanning during treatment. This value was compared with the I.C.R.U. bladder reference point dose rate calculated from lateral radiographs taken after insertion. The ratio of the maximum bladder base dose rate to the I.C.R.U. reference dose rate varied from 1.01 to 3.59. In ten patients the maximum bladder dose rate was not in the midline. Re-examination of five patients revealed significant changes in bladder base dose rate in two due to changes in applicator positioning and packing. The bladder base dose rate on the vertical plane through the middle of the vaginal ovoids was within +/- 25% of the maximum bladder base dose rate in 22/25 examinations.