There are so many patients with advanced gastric cancer who undergo systemic chemotherapy worldwide. The quality of life（QOL）of patients with gastric cancer who receive chemotherapy is often lowed by various gastrointestinal toxicities during the chemotherapy. Nutrition is also impaired by gastrointestinal toxicities. However, it is difficult to predict their occurrence in advance and further there is no good serum marker for nutrition in the patients treated with chemotherapy. Thus, it is important to objectively evaluate and predict the toxicity of the digestive tract during cancer chemotherapy. Diamine Oxidase（DAO）is an enzyme that is expressed in intestinal epithelial cells. Recently it has been reported that DAO activity may reflect damage or atrophy of the intestinal villi, and therefore it may be a sensitive serum marker for nutritional state. In this study, we measured serum DAO activity of patients with gastric cancer treated with systemic chemotherapy, and investigated the correlation between DAO activity and gastrointestinal toxicities. Six patients with gastric cancer, who were treated by docetaxel+cisplatin+S‐１combination chemotherapy, were enrolled. DAO activity was measured by sensitive colorimetric assay. DAO activities diminished after treatment in４patients with moderate to severe gastrointestinal toxicities. In contrast, they did not change in２patients with no gastrointestinal toxicities. Our results may suggest that DAO activity is a good serum marker for the gastrointestinal toxicities as well as nutrition state in patients who receive systemic chemotherapy. More large scale study is needed to warrant.