Cystic partially differentiated nephroblastoma.

An unusual cystic renal tumour is described which resembles, superficially, the solitary multilocular cyst of the kidney, but which shows appreciable amounts of proliferating undifferentiated renal blastema. We believe that this tumour represents a partially differentiated nephroblastoma and that, as such, it provides support for the theory that Wilms' tumour is capable of undergoing maturation to a fully differentiated, benign cystic lesion.