To date, no enclosure method for risk grouping patients with poorly-differentiated gastric adenocarcinoma has been identified. We examined the relationship between mutations in toll-like receptor 4 (TLR4) and patients with poorly-differentiated gastric adenocarcinoma. Genomic DNA was extracted from the peripheral blood of 38 patients, 20 with well-differentiated and 18 with poorly-differentiated gastric cancer, from 25 patients with colorectal cancer and from 10 healthy volunteers. The polymorphism of TLR4 up to the 2-kb upstream region of the 5' untranslated region (UTR) was analyzed. The results revealed the presence of single nucleotide polymorphisms (SNPs) only among patients with poorly-differentiated gastric adenocarcinoma. SNPs were found at 3 sites: -2081, -2026 and -1601 in 12, 15 and 15 of the 18 cases of poorly-differentiated gastric adenocarcinoma, respectively. The results of the determination of a consensus among the base sequences of the core promoter, basal promoter and upstream promoter elements reveal that the variant sites were present in the TLR4 mRNA promoter region, suggesting that they were biologically significant variations. Polymorphism analysis of the upstream region of the 5' UTR of TLR4 may be a useful new enclosure strategy for the risk grouping of poorly-differentiated gastric adenocarcinoma patients.