Endocrine neoplasms have been reported occasionally in patients with familial adenomatous polyposis (FAP). An adrenocorotical carcinoma was studied in a patient with a family history of FAP. Loss of heterozygosity (LOH) in the region close to the adenomatous polyposis coli (APC) gene was detected in this carcinoma, and evidence was obtained that there was a loss of the normal allele of the APC gene. This is the first demonstration of LOH at the APC locus in adrenocortical tumors. The present results and our previous data on LOH in a recurring desmoid tumor suggest that the heterozygous mutant/wild-type condition of the APC gene may give rise to benign tumors, and that functional loss of this gene leads to development of tumors not only in the colon but also in other various parts of the body in FAP patients.