Identification of the genes that are specifically expressed in either tumor or normal tissue is important for understanding cancer biology. Using differential displays, we obtained one gene which was specifically expressed in normal tissue but is only rarely expressed in carcinoma tissue of the human esophagus. The sequence of this gene was identical with cystatin B, known to be one of the cysteine-proteinase inhibitors, mainly inhibiting cathepsin L. There is little information on the clinical significance of cystatin B expression in human esophageal carcinoma. We thus studied the mRNA expression of cystatin B in 45 tumor/normal pair specimens of the esophagus, using the semi-quantitative reverse transcriptase polymerase chain reaction technique. The expression of cystatin B in tumor tissue was found to be markedly decreased compared with that of the corresponding normal tissue. The cases with a tumor/normal ratio of less than 0.5 showed high frequency of lymph-node metastasis and more advanced clinical stage as compared with those whose tumor/normal ratio was equal to or more than 0.5. The decreased expression of cystatin B protein in carcinoma tissue was confirmed by immunohistochemical staining. Our study suggests that reduced expression of cystatin B in esophageal-carcinoma tissue is associated with lymph-node metastasis and may therefore prove to be a useful marker for predicting the biologic aggressiveness of human esophageal carcinoma.