This study aimed to measure acoustic access to the prostate for extracorporeal ultrasound ablation. Both transabdominal and transperineal approaches were considered. The objective was to measure the size and shape of the aperture available for unobstructed targeting of the prostate. CT images of 17 randomly selected men > 56 years of age were used to create 3D reconstructions of the lower abdomen and pelvis. Rays were traced from target locations in the prostate toward the perineum and the abdomen. The maximum CT density encountered along each path was recorded; those paths that traversed structures with CT density exceeding a soft tissue threshold were considered to be blocked by bone. Unblocked rays comprised the accessible aperture. The aperture through the perineum was found to be a triangular-shaped region bounded by the lower bones of the pelvis varying significantly in size between subjects. The free aperture through the abdomen was wedge shaped limited by the pubis and also with great subject-to-subject variability. Average unblocked fractions of an f/1 transducer to target base, middle, and apex of the prostate along the urethra from the perineum were 77.0%, 94.4%, and 99.6%, respectively. Averages targeting from the abdomen were 86.1%, 52.3%, and 11.0%. Acoustic access to the prostate for therapy through the perineum was judged to be feasible. Access from the abdomen was judged to be sufficient for the base of the prostate, but likely inadequate for the middle and apex.