Familial adenomatous polyposis (FAP) is an inherited disorder caused by germline mutation of the adenomatous polyposis coli (APC) gene. Increased risk of hepatoblastoma (HBL) in FAP kindreds has been reported. To determine whether inactivation of the APC gene plays a role in development of HBL, 13 sporadic infantile hepatic tumors were analyzed for genetic alterations in the APC gene. A PCR-mediated RNase protection analysis was performed to detect subtle genetic alterations in the mutation cluster region and in exons 3 and 4 of the APC gene. The results showed that a G to T transversion at the splice acceptor site of the intron 3-exon 4 junction had occurred in one HBL. Sequence analysis of normal tissue of the patient proved the mutation to be germinal. Southern blot analysis at the APC locus revealed that the tumor had lost the opposite allele and was isodisomic at this locus. RNA analysis indicated that the tumor contained only the small APC transcript, from which exon 4 was entirely absent. Since abnormal splicing causes termination due to frameshift, it was hypothesized that only the truncated APC protein was expressed in this tumor. These findings suggest that inactivation of the APC gene is closely related to tumorigenesis of HBLs in FAP patients.