A large congenital mesoblastic nephroma (CMN) of combined classical and and cellular histological structure was removed from a 1-month-old female infant. The tumor extended extrarenally and may have been incompletely excised. Tumor tissue showed a mosaic hyperdiploidy with 54 chromosomes in the hyperdiploid line. No other antitumor therapy was given and there has been no recurrence after 4 years. Genomic imprinting normally prevents transcription of the maternal gene for insulin-like growth factor 2 (IGF2). Relaxation of IGF2 imprinting leading to abnormal transcription of the maternal gene is found in a majority of Wilms' tumors and in other malignant neoplasms. The biallelic transcription of IGF2 demonstrated in the CMN from this case is consistent with abnormal transcription of the maternal allele. Relaxation of imprinting of the maternal IGF2 gene or abnormal expression of the gene through other mechanisms may have a role in the genesis of CMN or the cellular subtype.