BACKGROUND:: The differential diagnosis in an elderly woman with a complex pelvic mass includes benign and malignant diseases. CASE:: A woman in her ninth decade was discovered to have both a breast mass and a pelvic mass at examination. After diagnostic breast biopsy results that confirmed invasive breast carcinoma, pelvic examination and ultrasonography were performed. The ultrasonogram demonstrated a large complex pelvic mass. The patient underwent an exploratory laparotomy at the time of her mastectomy. The pelvic mass was a protrusio acetabuli resulting from a prior right total hip arthroplasty. CONCLUSION:: A complex pelvic mass secondary to protrusio acetabuli is a rare clinical finding. © 2004 by The American College of Obstetricians and Gynecologists.