An abdominal mass was incidentally detected in a 12-year-old, neutered female, crossed breed dog. Abdominal ultrasonographic examination showed a well-delineated, irregular, heterogeneous mass that did not generate any distal acoustic shadowing. Transcutaneous US-guided biopsy of the mass were nonconclusive but raised the possibility of neoplasia. Surgery discovered a mass embedded in the omentum and a large quantity of surgical sponges were identified in cut section. To the authors' knowledge, this represents the first published case of gossypiboma casting no characteristic distal acoustic shadowing.