Congenital mesoblastic nephroma (CMN), first described by Kastner in 1921,1 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. It has an incidence of 2.8% of all renal tumors of childhood, with a mean age of presentation of 3.4 months, in contrast to an average age of 3 years in Wilms tumors.2 It has been documented as being 22.8% of all primary tumors in children 1 year old or less.3 A neoplasm in the kidney of a child less than 3 months old is usually a CMN. The majority of renal neoplasms originating in the fetus and found during the first weeks of life differ in structure and in biological behavior from a nephroblastoma. In contrast to cystic lesions of the kidney, solid renal neoplasms are rare in the newborn and account for only 8% of neonatal tumors. In the Children’s Cancer Group (CCG) neonatal study, there were 25 neonatal renal neoplasms, of which 17 were CMN and the rest were Wilms tumors.4 A review of neonatal Wilms tumors in the national Wilms tumor register identified 15 cases out of 6832 patients with an incidence of 0.16%, demonstrating how rare malignant renal neoplasms are in neonates. Although prenatal ultrasound is capable of detecting renal neoplasms in utero, there is no specific sonographic characteristic that can differentiate a CMN from a Wilms tumor. Both tumors present as a palpable abdominal mass in the neonate. Males outnumber females two to one with CMN, and both sexes are equally affected by Wilms tumors.