The ultrasound reflection characteristics in 5 patients with Wilms’ tumors are described. In the A-scope, the amplitude of the tumor echoes is not distinguishable from that of the kidney and liver, so that the boundary between tumor and renal or, if infiltrated, hepatic parenchyma cannot be defined. Necrotic foci within the tumor are probably the cause of the isolated high energy reflections. In the B-scan the Wilms’ tumor appears as a well defined, more (necrosis?) or less echo-free zone because of its very low impedance differences when the apparatus is optimally adjusted for the visualization of the kidney and liver. When the sensitivity is increased more than this, the weak echoes are amplified to the point where normal parenchyma and tumor tissue are no longer distinguishable from one another. While the value of ultrasound tomography can be considered as proven for the optimal planning of radiation therapy, the additional information from the simultaneous A-scope examination could possibly lead to a classification of the various differential diagnostic possibilities based on their internal structures. These further possibilities are implied here with the description of the known sound reflecting qualities of other space-occupying processes in the kidney region.