OBJECTIVE This study was undertaken to evaluate the efficacy of three-dimensional transrectal ultrasound to identify and measure the prostate zones. METHODS In an anatomic-sonographic study, eight specimens were investigated by means of three-dimensional transrectal ultrasound. Subsequently, the volumes of the prostate and the transition zone were measured by means of a special planimetric program; the images and results obtained were compared with anatomic sections and water displacement volume measurement. Then two groups of patients were investigated by means of three-dimensional transrectal ultrasound; the first group included 90 patients ranging in age from 55 to 85 years who presented with benign prostatic hyperplasia (BPH), which was verified by means of histologic examination. Three-dimensional transrectal ultrasound was performed prior to ultrasound-guided biopsies of the prostate. The second group comprised 10 patients aged between 17 and 30 years whose prostates were free of disease. RESULTS Three-dimensional transrectal sonography is the first imaging technique that can simultaneously demonstrate relevant structures in three planes; apart from the sagittal and the horizontal (or axial) plane, the region of interest can be examined in the coronal plane as well. The structural differences of the prostatic zones in BPH and the juvenile gland can be clearly identified with the help of three-dimensional transrectal ultrasound. The coronal plane provides important additional information; the central zone and the enlarged transition zone can be identified best in this third plane. CONCLUSIONS The prostate zones, their interrelations, and, in particular, the hyperplasia of the transition zone in BPH are clearly demonstrable. Furthermore, the size of the enlarged transition zone can be exactly measured.