Ultrasound scanning is useful in evaluating renal transplants: it is atraumatic, independent of renal function, and well suited for analyzing such common complications as lymphocele, abscess, hydronephrosis, and hematoma. When there is uncertainty as to the nature of a paranephric mass, ultrasonically guided percutaneous aspiration is definitive. Ultrasound can also detect the acute increase in renal volume associated with rejection and may prove to be a valuable adjunct in the early diagnosis of this problem.