With the advent of 3-dimensional computed axial tomography simulators (CT sim), more sophisticated conformal radiation treatment planning is possible. In order to estimate the accuracy of prostate volumes calculated by C T sim, the volumes obtained by C T sim and ultrasound (US) and diagnostic C T were compared. From April 1989 to December 1992, 212 prostate cancer patients were treated with conformal techniques guided by diagnostic CT. Fifty of them had their prostate volume determined by transrectal ultrasound (US). The prostate volumes calculated using diagnostic C T and US were compared. From April 1993 to May 1994, 186 patients were treated with conformal technique guided by C T sim. Eighty-three of them had their prostate volumes determined by ultrasound. The prostate volumes were determined using the ellipsoid model for each of the imaging methods (ultrasound, diagnostic CT, C T simulator). An additional integral method was available for determining volume with the C T simulator. The median ellipsoid model volume of the prostate gland was 40.3 cc by US and 54.4 cc by diagnostic CT. Correlation coefficient (8) was 0.608 (P < 0.001). C T sim: The corresponding median ellipsoid model volumes were 37 cc by US, and 49.8 cc using the ellipsoid model by C T sim. The C T sim volume was 38.1 cc using the integral method. The 8 of the prostate volumes by US and C T sim (integral method) was 0.803 (P < 0.001). The prostate volume calculated using the integral method by C T sim was similar to the volume obtained using the ellipsoid model with US. The ellipsoid model method of calculation was significantly different from the ultrasound volume for both the diagnostic and C T sim method. The ellipsoid model calculation for US and the integral volume calculation for C T sim most closely approximates true gland size.