The purpose of this study was to reevaluate the significance of serum PHI in gastrointestinal cancer at histopathologically defined stages prior to primary treatment. A total of 248 patients with malignant tumors of the gastrointestinal tract and a collective of 42 patients with noncancerous diseases were studied. The results are compared with those obtained with the established markers tissue polypeptide antigen (TPA) and carcinoembryonic antigen (CEA). Phosphohexose isomerase (PHI) revealed an overall diagnostic sensitivity of 69%, combined with a specificity of 74%. The corresponding data for TPA were found to be 73 and 47% while for CEA 26 and 95% respectively were determined. Even in the early stages of colorectal and esophageal carcinoma, PHI showed a sensitivity of about 60%. A continuous rise of PHI serum levels, correlating well with the extent of the tumor disease, could be detected. In contrast to TPA and CEA, PHI assay can be carried out with a minimum of laboratory efforts, in a short time and at low costs. These findings suggest that serum PHI assay is a useful aid for screening of gastrointestinal cancer, especially esophageal and gastric carcinoma, and a reliable marker for treatment control and follow-up.