The aim of the present study was to examine the significance of the p21 expression in gastric cancer. We examined the expression of p53, p21, TGF beta 1 and PCNA in 75 cases of gastric cancer using immunohistochemical examinations and the expression of p21 RNA by in situ hybridization (ISH). The combination of p53 and p21 expression was related to depth of invasion, lymph node metastasis, and stage grouping. The survival curves of the p53 negative-Group and the p21-positive Group were significantly higher than those of the p53-positive and the p21-negative Group, the p53-and-p21-both-positive Group, and the p53-and-p21-both-negative Group (each p < 0.01). The average PCNA Labelling Index (LI) of the p53-negative-and-p21-positive Group was significantly lower than that of either the p53-positive-and-p21-negative Group or the p53-and-p21-both-positive Group or the p53-and-p21-both-negative Group (p < 0.01, p < 0.05, p < 0.05, respectively). All of the p53-and-p21-both-positive cases were TGF beta 1 positive, and the rate of the TGF beta 1 positive cases in the p53-and-p21-both-positive Group was significantly higher than that of the p53-positive-and-p21-negative Group, and than the rate in the p53-and-p21-both-negative Group (each p < 0.01). The survival curves of the cases with expression of p21 RNA were higher than that of cases without p21 RNA (p < 0.05). Many of the p53-positive-and-p21-negative cases were advanced cancer with very poor prognosis, but many of the p53-negative-and-p21-positive cases were early cancer with good prognosis. These results suggest that p21 suppressed synthesis of DNA via PCNA, and TGF beta 1 is a regulation factor for the expression of p21, and that the combination of p53 and p21 expression is concluded to be a useful prognostic marker of gastric carcinoma.