A 57 element aperiodic linear phased array was designed and constructed to investigate the feasibility of using transrectal ultrasound for the thermal therapeutic treatment of prostate cancer and benign prostatic hyperplasia. A method of reducing grating lobe levels by using optimized random distributions of unequally sized elements is introduced. Using this technique, array periodicity is avoided, making it feasible to use larger elements and hence fewer elements and amplifier channels, while still achieving acceptable power field patterns. Acoustic power field simulations determined that the grating lobe levels associated with selected aperiodic element distributions were approximately 30%-45% less than those associated with periodic element spacing and the same average element width. Or by using aperiodic rather than periodic element distributions, the average element width could be increased by approximately 20%-35% (approximately lambda/4.4), while maintaining a constant grating lobe level. Prior to construction of the 57 element array, the power capabilities of this type of array were demonstrated with a 16 element aperiodic phased array, which delivered over 28 W of acoustical power per cm of array length while focused. The power field patterns produced by the 57 element array closely matched the field patterns predicted by the theoretical model used in the simulations. The array produced acceptable power field patterns for foci at depths up to 5 cm and up to 2 cm off the center axis, in addition to producing multiple foci simultaneously. Based on the power capabilities and field patterns, this aperiodic array design has the potential to be incorporated into a clinical heating device as a means of delivering thermal therapies to the prostate and other target volumes close to body cavities.