Sonography of the pelvis is widely used for the diagnosis of masses in both females and males. Cystic and solid masses can easily be differentiated by ultrasound. Cystic lesions in the pelvis are related to the genitourinary system or the gastrointestinal tract. In the past 30 years, erectile impotence has been treated with inflatable penile prostheses. The following two case reports are of the sonographic appearance of a cystic collection in the pelvis that in fact was the reservoir of an inflatable penile prosthesis. CASE REPORT 1A 44-year-old white male with a past medical history of insulin-dependent diabetes and hypertension presented with a l-week history of increasing right costovertebral angle tenderness radiating to the right lower quadrant and pelvis. The patient also had low-grade fever. Ultrasound examination of the abdomen and pelvis revealed a 4-cm hypoechoic mass with good sound transmission located anterior and lateral to the right side of the urinary bladder (Fig. 1). The mass was not communicating with the bladder. The differential diagnosis that was entertained at the time of the examination was a bladder diverticulum or a mass of inflammatory etiology such as an appendiceal abscess or a diverticular abscess. On further questioning of the patient, it was learned that the patient had an inflatable penile prosthesis. A plain film of the abdomen (Fig. 2) confirmed the presence of the prosthesis and the reservoir in the right hemipelvis, which gave the sonographic appearance of a cystic mass. CASE REPORT 2A 55-year-old Hispanic male with a history of laminectomy had an abdominal ultrasound examination because of right upper quadrant pain and right lower quadrant discomfort.This study showed gallstones. Also noted was a cystic mass in the right side of the pelvis that measured 4.5 cm x 3 cm (Fig. 3). The mass was not communicating with the bladder and did not change with voiding. The patient had had implantation of a penile prosthesis for erectile impotence. Being familiar with the cystic appearance of a penile reservoir, we were confident that the cystic mass in this patient represented the reservoir, and this was confirmed by pelvic x-ray. DISCUSSIONErectile impotence is a common complaint of men with diabetes mellitus, postoperatively following prostatic or urethral surgery, and in cases of spinal injuries. In the past 30 years the use of the inflatable penile prosthesis to treat erectile impotence has become an established method of treatment. In 1973, Scott and co-workers' developed a penile prosthesis that could be inflated and deflated. The prosthesis consisted of two hollow silicone cylinders connected to a pump, which in turn was connected to a reservoir (Fig. 4). The reservoir is implanted into the rectus muscle in the perivesical space. It is filled with radiographic contrast medium. The patient activates the prosthesis by squeezing the pump, which is implanted in the scrotum, forcing fluid into the silicone cylinders. Evaluation of the prosthesis can be made with abdominal radiography.2...