Apoptosis of the target cells is an important index in the assessment of the efficacy of cancer chemotherapy. We previously established a new experimental technique in which cancer cells are distributed in thin collagen gel as one or two cell layers, and cultured with anti-cancer drugs. The cells are stained with fluorescent Hoechst 33258 (Ho) and photographed, then with hematoxylin and eosin (H&E) and again photographed. The results show that most cell death patterns can be determined by combining observations of Ho and H&E-stained cells without the necessity for judging the apoptosis by electron microscopy. 5-fluorouracil (5-FU) and cisplatin (CDDP) are important anti-cancer drugs in the treatment of a variety of cancers. 5-FU or CDDP alone have shown significant effects in the treatment of gastric and colon cancers, and in addition, it has been shown that the combination of 5-FU plus CDDP (FP) therapy produces synergism greater than 5-FU or CDDP alone in gastrointestinal cancer. In this study, we evaluated the efficacy and toxicity of FP therapy in the gastric cancer cell lines MKN45, MKN28, and KATOIII and the colon cancer cell lines HCT116 and COLO320, and examined the relationship between the response to FP therapy and apoptosis. Additionally, we performed transfection of normal p53 gene into p53 mutant MKN28 cells and analyzed the impact of the p53 gene on a sensitivity test. Wild-type p53 in MKN45, HCT116, and COLO320 cells underwent significantly (p<0.01) more apoptosis than MKN28 and KATOIII cells possessing p53 mutant- and deficient-type, respectively, in FP therapy. Transfection of p53 to MKN28 cells resulted in a significantly (p<0.01) higher apoptotic index. From these results, we conclude that the p53 pathway allows induction of apoptosis in gastrointestinal cancers in FP therapy treatment, and that identification of the p53 type of a patient's cancer can be used to predict the success of FP therapy.