As surgery has developed, the salvaging of functioning tissue has received increasing attention, and nowhere has this been more strikingly demonstrated than in the operative attack on the kidney. The parenchyma preserved by renal resection is both vitally precious and irreplaceable; it undergoes hypertrophy but not regeneration. The number of patients whose existence is attributable to renal parenchyma left by conservative resection is constantly growing. Developmental anomalies occur more often in the urinary tract than in any other system, and in at least 80 per cent of the cases subjected to renal resection the principal urologic disease involves an anomalous upper urinary tract. In most instances there is pelvic reduplication with one pelvis involved by hydronephrosis, usually infected and often complicated by stone. As a corollary, the discovery of resectable kidneys will be notably increased by greater accuracy in the diagnosis of anatomic structure and the finer localization of chronic