The Authors pass on the case of a nephroblastoma, associated to Wilms Tumorlet and combined nephroblastomatosis, arose on a multicystic dysplastic kidney. They examine the relationships between the nephroblastoma and the kidney malformations, the possibility of malignant degeneration of a multicystic dysplastic kidney and the necessity of the nephrectomy as a prevention of the degeneration. At the moment the statistic data don't justify the nephrectomy a the birth to prevent the arising of a nephroblastoma.