In the present fluorescence in situ hybridization (FISH) study of six congenital mesoblastic nephromas (CMNs) using ETV6 and NTRK3 probes as well as a chromosome 15 painting probe, we identified a cryptic reciprocal translocation, t(12;15)(p13;q26), in one tumor, and an insertion, ins(12;15)(p13;q22q26), in another that were not previously identified by cytogenetic analysis. An interphase FISH study with the same probes detected the ETV6-NTRK3 fusion signal in all three cellular or mixed type tumors, but not in all three classical type tumors. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis detected the ETV6-NTRK3 fusion transcript in the three cellular or mixed type tumors, but not in the three classical type tumors. FISH analysis using a chromosome 11-centromere probe detected trisomy or tetrasomy 11 in all three tumors with the ETV6-NTRK3 fusion signal. To clarify whether IGF2, a paternally expressed gene on chromosome 11, has a certain role in the tumorigenic process of CMN through a loss of imprinting (LOI), we studied IGF2 allelic expression. We found no LOI in two cellular or mixed type tumors or in two classical type tumors, and concluded that the role of the LOI of IGF2 is not essential for the development and progression of CMN with or without trisomy 11. Furthermore, we showed no rearrangements of the MLL gene, which is frequently rearranged in acute leukemia with +11 in the three CMN tumors with +11.