Congenital mesoblastic nephroma (CMN) is an unusual renal tumor, similar in some respects to Wilms' tumor and reported only in infants. We report a case of CMN in a 2-year-old boy. He was referred to us with a right abdominal mass. Excretory urogram, arteriogram and computed tomogram revealed a large malignant renal tumor on the right side. With a clinical diagnosis of Wilms' tumor of the right kidney, he received radiation and chemotherapy. After radiation and chemotherapy, tumor size was decreased. Right nephrectomy was performed on April 22, 1982. The kidney and tumor weighed 270 g and measured 12 X 7 X 5 cm. The tumor extended to the renal pelvis and the ureter with polypoid growth pattern. Microscopically, the tumor was composed of compact interlacing bundles of elongated fibromyomatous cells. Islands of entrapped glomeruli and tubules were also detected within the tumor. He was well and had no signs of recurrence of tumor at the age of 3 years
