The accumulation of mutated p53 in tumor cells results in the presence in the circulation of mutant p53 (p53m) and the production of p53-specific antibodies (p53Ab). We examined the relationships among these phenomena and analyzed their clinical implications in 62 patients with gastric cancer at various stages. Expression of p53 in tumors was studied by an immunohistochemical method and circulating p53m and p53Ab were quantitated with commercially available enzyme-linked immunosorbent assay kits. The detectable expression of p53 in tumors and circulating p53Ab was recognized in 28 (45.2%) and 7 (11.3%) of the 62 patients, respectively. The number of patients with higher levels of circulating p53m increased with the progression of the depth of cancer invasion. Patients with any positive findings for the three p53-related parameters had a poorer prognosis, and the difference was statistically significant in patients with p53Ab. When survival was analyzed in terms of the combination of the three p53-related parameters (detectable expression of p53 in tumor cells, high levels of p53m and p53Ab in the circulation), a significantly poorer prognosis was associated with an increase in the number of positive parameters. Analysis of p53 in tumor cells, together with analysis of circulating p53m and p53Ab, could improve the accuracy of prognosis in patients with gastric cancer.