The concentrations of free amino acid and protein-bound amino acid in gastric and colon cancers and also in normal tissues were measured and compared in order to investigate the differences in amino acid metabolism between these cancers. The examined materials were 20 specimens of gastric cancer, with 20 specimens of normal gastric full-thickness layers and 11 specimens of normal gastric mucosal layers, as well as 16 specimens of colon cancer, with 12 specimens of normal full-thickness layers and 16 specimens of normal colon mucosal layers. The amino acid pattern in gastric cancer was more similar to that in the gastric full-thickness layer than the gastric mucosal layer. On the other hand, the pattern in colon cancer was more similar to the colon mucosal layer than the full-thickness layer of normal colon tissue. Comparison between gastric and colon cancers showed that almost all of the protein-bound amino acid levels in gastric cancer were significantly higher than in colon cancer, while, conversely, all of the free amino acid levels except for aspartate, glutamate and glutamine were significantly lower in gastric cancer than in colon cancer. Both in the stomach and colon, the aspartate and glutamate levels in cancer tissues were significantly higher than in normal full-thickness or mucosal layers.