The usefulness of chemosensitivity testing of cells collected by endoscopic biopsy using the adenosine triphosphate assay (ATP assay) was investigated for esophageal tumors. Correlation between this chemosensitivity test and other chemosensitivity tests was more than 80% in most combinations. The predictive rate of clinical sensitivity was 77.8% and of clinical resistance was 68.8%. The predictive accuracy was 72.0%. These results will extend the indication and usefulness of chemosensitivity testing in inoperable and preoperable cases.