Retroperitoneal iliopsoas bursal extension of hip joint disease has been described by Finder (1) and Stephens (2). Such spread may be overlooked in the differential diagnosis of abdominal, pelvic, and inguinal masses. We have observed three patients in whom retroperitoneal extension of hypertrophic villonodular synovitis of the hip via the iliopsoas bursa simulated other abdominal and pelvic lesions. A triad or syndrome was observed, consisting of a palpable mass, extrinsic pressure on adjacent structures, and x-ray changes in the hip characteristic of advanced degenerative or destructive arthritis. In one case, the bursal extension from the right hip extended into the abdomen and displaced the cecum and small intestines. In the second, the distended iliopsoas bursa produced medial displacement of the lower ureter and sigmoid and clinical evidence of compression of the external iliac and/or femoral vein. In our third case a bulging bursal mass anteriorly was believed on separate occasions to represent an undescended testicle, lymphadenopathy, aneurysm of the femoral artery, inguinal and femoral hernias. Case Reports Case I: A. T., a 76-year-old woman, was admitted to Deaconess Hospital Feb. 12, 1964, with a large mass in the right lower abdomen, discovered by her physician about one month prior to admission. The patient had experienced occasional abdominal discomfort for a few months and had been bedridden and unable to walk since 1935 because of severe rheumatoid polyarthritis. Steroid and aspirin medication had been administered from 1958 to 1960. A fairly large, smooth, moderately fixed mass measuring about 12 cm in length was palpable in the right lower quadrant of the abdomen. Roentgen examination showed a large homogeneous soft-tissue density or mass in the right lower quadrant of the abdomen and right side of the pelvis (Fig. 1), “radiating” out from the hip region. Absorption of the right femoral head was evident. The roof and medial portions of the acetabulum were deformed, the latter as a result of fracture in 1961. Barium-enema examination demonstrated marked upward and medial displacement of the cecum and small intestines. Intravenous urography failed to outline the ureters. The clinical diagnosis was ovarian cyst, possibly malignant. Exploratory laparotomy on Feb. 14 disclosed in the right side of the pelvis and lower abdomen a large retroperitoneal cystic mass. This measured about 15 cm in diameter and was dissected free down to the point where the right iliopsoas muscle emerged from the pelvic cavity. Compression of the cystic mass resulted in bulging over the anterior aspect of the hip. Therefore, the joint was exposed through an anterior incision, and the excess synovial tissue and bursa were removed retrograde. The major section of tissue submitted for pathological examination consisted of a flattened, purplish-tan fragment measuring 20 × 6 × 1.5 cm.