We used a personal computer and commercially available software to display prostatic contours of 5 normal prostates, 10 with hypertrophy and 5 with cancer, in three dimensions, based on transrectal ultrasonograms. Each had a distinctive shape. The contour of a prostatic cancer had an irregular surface, while that with prostatic hypertrophy was smooth and spherical; a normal prostate was flat. Time required to input the prostatic contours of one case was about thirty minutes, however, long enough to prevent three-dimensional display of the prostate from becoming a routine procedure. Twenty three-dimensional prostatic models thus made, however, were used to develop a system to improve the accuracy of the sonometrics of the prostate by calculating ellipsoid volume. With this system, the operator chooses from among 20 prostatic models displayed on a monitor the one whose cross and sagital sections most closely resemble those of the prostate being diagnosed. The system then shows the degree of error of ellipsoid volume calculation of the chosen three-dimensional prostatic model. This procedure was more practical in that it required only about three minutes.