OBJECTIVE To compare the volumes and dimensions of the prostate gland as measured by transrectal and transurethral ultrasonography (TRUS and TUUS) and to study the prostatic changes that occur in the presence of an urethral instrument. METHODS Twenty men (mean age 71 years, range 43-85) with symptoms of prostatic enlargement underwent TRUS and the dimensions and volumes of their prostates were obtained by the dimensional method and by step planimetry. Within 24 h, all the men were examined cystoscopically under sedoanalgesia and underwent TUUS. Their prostatic volumes and dimensions were again measured by the dimensional method and by step planimetry. Step planimetry was carried out using a specially designed indexer firmly attached to the examination couch or operating table. All the static images and planimetry slices were video recorded for later computer enhancement and to study the three-dimensional changes occurring in the prostate. The volumes and dimensions obtained by TRUS and TUUS were compared. RESULTS Volumes obtained by transurethral step planimetry were 22% greater than those from transrectal planimetry. Although there was a good correlation (r = 0.984) between transurethral planimetry and the volumes derived using the transurethral dimensions with the prolate ellipsoid formula, the latter produced values 17% and 25% lower by the transurethral and transrectal routes, respectively. These volume estimates varied widely, indicating that the simple addition of a constant to the prolate ellipsoid formula would not correct the volume. Three-dimensional changes of the prostate occurred with TRUS and TUUS: with TUUS, the craniocaudal and anteroposterior diameters were significantly larger (by 7% and 18%, respectively) while the transverse diameter was smaller by 20%. CONCLUSIONS The estimated volumes and dimensions of the prostate differed when measured by TRUS and TUUS and three-dimensional changes in the prostate occurred in the presence of an urethral instrument.