Wilms' tumor (WT) is an embryonal renal malignancy, which overexpresses insulin-like growth factor II (IGF-II), a fetal mitogen. Relaxation of parental imprinting of IGF2, the gene encoding IGF-II, is found in Wilms' tumors, suggesting an important role for IGF2 dosage in tumorigenesis. The IGF2R gene encodes a nonmitogenic receptor which targets IGF-II to the lysosomes for degradation and, therefore, inhibits the mitogenic function of IGF-II. The human IGF2R is imprinted in a proportion of normal individuals. To test the hypothesis that IGF2R imprinting predisposes to Wilms' tumor through the effect of decreased IGF2R dosage on IGF-II inactivation, we examined IGF2R imprinting in Wilms' tumors. Two transcribed CA repeat polymorphisms were used to distinguish the two alleles in the RT-PCR product. We observed that in 7/16 of Wilms' tumor patients, the paternal IGF2R was markedly but not completely repressed in both tumor and normal kidney. In one additional case, IGF2R was likewise imprinted in the tumor but not in the normal kidney. A similar imprinting was observed in fetal tissues and placenta prior to 20 weeks fetal age but not in term placenta or postnatal blood cells, indicating abnormal persistence of a fetal pattern in the kidneys of Wilms' patients. Genetic analysis showed association of the imprinting with a cis-acting locus. The high frequency of aberrant persistence of IGF2R imprinting in the kidneys of Wilms' tumor patients, which may be an embryonic feature, suggests that it is a predisposing factor in tumorigenesis. This is in accordance with evidence that IGF2R is a tumour suppressor in other types of malignancies.