Loss of heterozygosity (LOH) at APC and MCC gene loci (both mapped to 5q21) was investigated in 24 surgical specimens of primary gastric carcinomas using the polymerase chain reaction after tumor cell enrichment by cell sorting based on differences in DNA content. LOH at APC and/or MCC was detected in 87% (13/15) of the cases; at the APC in 86% (12/14) and at the MCC locus in 100% (7/7). LOH at the APC locus was always accompanied by LOH at the MCC locus. LOH at the APC and/or MCC was found in both differentiated and undifferentiated types in both early and advanced stages of gastric carcinoma. Thus, LOH at APC and/or MCC is considered to be one of the most prevalent genetic alterations in human gastric carcinoma and occurs at an early stage of the carcinogenesis.