Ultrasound scans in patients with nonopaque filling defects of the renal pelvis were analysed. Ultrasound can differentiate between nonopaque stones (stronger echo and no shadow) and other causes of filling defects (blood clots and tumors: and strong echo, no shadow). Tumor and blood clot are only to differentiate by the movement and transitory nature of the blood clot. Experimental studies with cadaver kidneys shows that the acoustic shadow behind gas in the renal pelvis (due to fistula etc.) has indistinct margins and reverberation echoes, which contrasts with the distinct acoustic shadow found with calculi.