The authors report a case of aneurysmal bone cyst in a female, revealed by a pelvic mass. Ultrasonography visualised the pelvic mass, its fluid content, internal septations and fluid level and displacement of surrounding structures. Sacral plain radiographs and computed tomography images showed trabeculated osteolysis involving the sacrum, sacro-iliac joint and adjacent iliac wing. The endopelvic component had a liquid-density varying between 15 and 30 HU, with contrast enhancement of its wall and internal septations.