ULTRASOUND in renal transplantation studies may be used for (1) initial pretransplant evaluation in both the donor and the recipient, (2) immediate postsurgical examination, and (3) long-term serial follow-up. The ability of ultrasound to analyze soft tissue, both within and surrounding the kidney, makes it an important complementary examination to the functional information provided by an excretory urogram. An understanding of the ultrasound characteristics of the kidney are important, since changes in these echo patterns are suggestive of disease. The kidney has strong echoes that arise from the outer surface—the capsule. Strong echoes are also produced from the midportion of the kidney and are caused by numerous reflecting interfaces from the renal hilus, defined as the central collecting system, which includes the blood vessels, the fatty fibrous tissue, and the pelvicalyceal system. While any of these structures may become prominent, a separation of the central collecting system usually implies hydronephrosis.