The progressive proportion of 25-50 and 75% of undifferentiated cells in the Wilms' tumor mass generate three grading categories: well differentiated (WD); the moderate differentiated (MD) and the poorly differentiated (PD). The Urology Department of the Instituto Nacional de Enfermedades Neoplásicas (Lima, Peru) has studied 60 selected cases registered at its Biostatistics Department. Only 6.9% were well differentiated; 69% were of the poorly differentiated type. In 58 patients our survival rate at two years after diagnosis is higher (39.53%) when the patients are over 2 years of age; while under 2 years of age this survival rate is 26.67%. In a group of 46 patients, the survival rate at two years after initial diagnosis is 44% with a tumor size between 10-14 cm; the survival rate drops to 39.3% when the tumor size is over 15 cm in diameter. There is a linear increasing scale of survival rate for the well differentiated (25%), moderate differentiated (27.3%) and for the patients bearing poorly differentiated types of tumors (48.4%). Further studies are encouraged in order to learn the reasons for this correlation.