A multileaf collimator (MLC) is compared with conformal blocks for delivering the boost phase of dose-escalated conformal prostate radiotherapy. When using conformal blocks, the volume of rectum irradiated to 90% (V 9 0 ) is lower (1.4+/-1.3%, 1 SD) for a three-field plan with gantry angles 0 o , 90 o , 270 o than for a six-field plan with gantry angles 50 o , 90 o , 130 o , 230 o , 270 o , 310 o (2.1+/-1.3%, P=0.002). However, when using an MLC in which the leaves and wedge are oriented at right angles, V 9 0 is higher (4.7+/-3.0%) for a three-field plan than for a six-field plan (2.7+/-1.6%, P=0.05). The larger increase in V 9 0 for the three-field plan when changing from conformal blocks to MLC is mainly due to the limitation imposed upon the MLC orientation by the use of wedges.