Flow cytometric analysis of nuclear DNA content was performed on excised malignant tissue of the esophagus. The DNA ploidy pattern was compared with a variety of histologic parameters and the subsequent clinical course to determine whether or not this pattern is associated with the mode of malignant potentiality. Of the 31 patients, eight had the diploidy DNA pattern and 23 the aneuploid DNA pattern. Tumors with the aneuploidy DNA pattern had a significantly higher frequency of lymph node metastasis than did those with the diploidy DNA pattern (P less than 0.01). Mitotic rates in the aneuploid tumors were significantly higher than was the case in diploid tumors (P less than 0.0005). The incidence of recurrence within 12 months after surgery was higher in patients with aneuploid tumors (83.3%) than in those with diploid ones (16.7%), with a statistical difference (P less than 0.05). Thus, the DNA aneuploidy based on flow cytometry closely correlates with the high frequency of nodal involvement and high mitotic rates, factors generally indicative of the aggressive behavior of the malignant tumors. DNA aneuploidy based on flow cytometric analysis is a pertinent index for determining the highly malignant potential in esophageal carcinoma.