The 11p15.5 chromosomal region contains one or more loci involved in congenital developmental abnormalities and in the genesis of embryonal tumors, such as Wilms' tumor, embryonal rhabdomyosarcoma, and hepatoblastoma. In these tumors, a loss of constitutive heterozygosity, selectively involving a specific parental allele, suggests both the presence of onco-suppressor genes and a phenomenon of genomic imprinting. We present evidence that both genetic events could be occasionally involved in hepatoblastoma. In fact, loss of heterozygosity at 11p15.5 could be documented in 3 of 13 patients with hepatoblastoma, and in 2 cases the paternal origin of the residual allele in the tumor was assessed. Moreover, imprinting of the paternal IGFII allele and the maternal H19 allele was confirmed in normal tissues of 5 informative patients. Finally, imprinting relaxation of IGFII was detected in the tumor tissue of 1 patient.