Congenital mesoblastic nephroma (CMN), first described by Kastner in 1921, is the most common renal tumor in the neonate, although rare cases present in later childhood. It is also known as a fetal renal hamartoma, mesenchymal hamartoma of infancy, or lipomyomatous hamartoma. Tumor types have irregular interdigitating margins in the perirenal fat, and wide margins of excision are desirable for complete removal. Incomplete removal results in tumor recurrence, which happens within a year of resection in most instances. The newborn usually presents with a large, nontender abdominal mass. Maternal polyhydramnios and prematurity are frequently seen, although the reason for this is unclear. Detailed antenatal ultrasound scans may pick up a solid tumor of the kidney. Plain films of the abdomen show a large, soft-tissue abdominal mass that is rarely calcified. Blood samples are obtained for a full blood count, group, and crossmatch. Careful monitoring and control of blood pressure is required to prevent dangerous perioperative fluctuations.