Human esophageal cancer is a common occurring malignancy with high mortality rate partially due to lack of tools for early diagnosis. In this study, we have analysed tumour tissue from 50 cases of primary esophageal cancer. Our studies showed that the activity of monoamine oxidase (MAO) and the expression of MAO-A were strikingly decreased in the tumour tissues of 48 (96%) and 44 (88%) patients, respectively. These results suggest that the activity of MAO and the expression of MAO-A may be used as new diagnostic markers for esophageal cancers.