The vast majority of renal neoplasms in the first weeks of life are benign, representing a fetal mesenchymal hamartoma. Three cases are reported, two in the newborn period and the third in a 2 month old infant. Although the tumor can be pathologically distinguished from Wilms’ tumor, the roentgenographic studies (including umbilical angiography and excretory urography) cannot make this distinction. Therapy of such hamartomas should be limited to surgical excision with avoidance of potentially harmful effects of chemotherapy and radiation therapy.