A 28-year-old woman with a giant cell bone tumor of the right distal femur was bedridden due to a pathologic fracture. She had overflow urinary incontinence. On Tc-99m MDP bone scintigraphy, crescent-shaped activity was seen in the bladder. Ultrasound showed mobile echogenic precipitate in the bladder. Urine analysis taken by cystoscopy revealed that the precipitate consisted of amorphous phosphate crystals.