['To compare the results of treatment planning with or without bladder contrast during simulation of three dimensional conformal radiotherapy (3D-CRT) for prostate cancer (18 MV X-rays, six field arrangement), and to assess the potential changes in dose distribution in the target and rectal volumes.', 'Based on CT-simulation using intravenous contrast media, 3-D conformal treatment planning was performed in five patients. To simulate a non-opacified bladder, the electron matrix density of the opacified bladder was virtually changed to water density. Two treatment plans were carried out, with and without bladder opacification. In each patient dose distributions were formally compared for both plans, and the increment in monitor units (MU) needed to compensate for the presence of contrast media was assessed.', 'A mean dose variation of -0.03% (range, -0.03-0.14%) and -1.13% (range, -1.85-0%) was observed for the prostate and the rectum, respectively. The average mean MU increment without bladder contrast normalized to the case with bladder contrast was 0.31% +/- 0.52.'] Does bladder opacification significantly influence dose distribution in conformal radiotherapy of prostate cancer?