Hepatoblastoma is the most common primary malignant liver tumor in children, yet little is known about molecular genetic changes in these tumors. Previous studies report loss of heterozygosity on chromosome arm 11p in some hepatoblastomas. We used the polymerase chain reaction to amplify multiple microsatellites on chromosome arm 11p to assess loss of heterozygosity in 18 hepatoblastomas. Loss of heterozygosity on 11p was found in six of them. The common region of overlap was restricted to the telomeric portion of 11p (11p15.5) and therefore excluded the WT-1 tumor suppressor gene at 11p13. Parental origin of the lost allele could be determined in all six cases and was exclusively maternal. These results indicate that a tumor suppressor gene at 11p15.5 is involved in the pathogenesis of hepatoblastoma and also suggest that this chromosomal region is imprinted.