Leiomyosarcoma of the seminal vesicle is exceedingly rare. The ultrasound and CT findings in a recent case are described. These consist of a mass causing enlargement of the right seminal vesicle with involvement of the urinary bladder and prostate. The prostate gland was displaced inferiorly and to the left. Surgical resection and subsequent histology confirmed the diagnosis. The role of imaging in retrovesical masses is also discussed.