Renal pseudoenlargement due to masses in posterior pararenal space

Two patients presenting with acute flank pain had unilateral smooth enlargement of a kidney on excretory urography with associated attenuation of the collecting system. This presentation suggested intrinsic renal disease, but sonography showed the urographic findings to be due in each case to a mass in the posterior pararenal space. By displacing the kidney anteriorly and away from the x-ray film, such masses will result in radiographic enlargement of the renal outline, i.e., renal pseudoenlargement. The distinction between true renal enlargement and pseudoenlargement may be made by sonography.