We studied the expression of insulin-like growth factor II (IGF2) and Wilms' tumor gene (WTI) in nine cases of congenital mesoblastic nephroma (CMN) and five cases of first trimester fetal kidneys by in situ hybridization. Our aim was to determine their site of expression and to correlate their histogenetic relationship to those of other childhood renal tumors. Our results showed that all nine cases of CMN (classic, mixed, and cellular) contained abundant IGF2 but not WT1 transcripts. The IGF2 transcripts were diffusely distributed over the tumor cells. These findings suggest that CMN is derived from primitive mesenchymal nephrogenic cells and have a potential to differentiate into a stromal cell lineage.