In a series of 889 Wilms' tumours we found 29 pure mesoblastic nephromas. The age of the patients varied from newborn to 22 months, but only five were older than four months. Two histologic types were recognized--leiomyomatous (9) and cellular (20) with a fibrohistiocytic appearance. The leiomyomatous type was almost invariably in stage I, smaller and present in younger infants. At operation half of the cellular type has ruptured or infiltrated the renal pelvis or perirenal tissue. Two patients died of postoperative complications. All others are alive after four years and free from disease.