It is currently accepted that colorectal tumorigenesis results from accumulation of multiple mutations in certain genes. This concept prompted us to search for possible mutations in the APC, k-ras, and p53 genes in an advanced cancer coexisting with a large villous adenoma of the rectum in a 54-year-old patient with no family history of colorectal cancer. Genomic DNA extracted from multiple subregions of the tumor and surrounding normal mucosa was studied by polymerase chain reaction (PCR) followed by single-strand conformation polymorphism (SSCP) analysis and direct sequencing. Both the adenoma and carcinoma had abnormal PCR-SSCP for APC (exon 11) and k-ras, irrespective of the location within the tumors. However, p53 abnormality (exon 7) was detected only in samples taken from the carcinoma. Subsequent sequencing revealed a TTG to TAG mutation at codon 479 of APC, a GGT to GAT mutation at codon 12 of k-ras in both the adenoma and carcinoma, and a CGG to TGG mutation at codon 248 of p53 (exon 7) in the carcinoma. These findings were in accord with the current concept of colorectal tumor progression whereby genetic alteration of APC and k-ras occurs relatively early while that of p53 is rather late and is possibly a decisive event in relation to malignancy.