This study was made to show the effect of stomach cancer on chromosome by sister chromatid exchange (SCE) analysis, because very few studies have reported. Metaphase preparation from peripheral blood lymphocytes of 12 patients were harvested before operation, along with those of 10 healthy control subjects. Mean SCE frequency in the stomach cancer patients was 7.0107+/-1.5720 as compared with 5.8267+/-0.7646 in controls. SCE values of patients with stomach cancer were significantly higher than those of control (p<0.035). This results suggest that patients with stomach cancer show a degree of chromosomal instability that might be related to a predisposition to neoplasia.