Microsatellite instability (MSI) caused by the defective functions of mismatch repair genes plays an important role in the carcinogenesis of gastrointestinal tumors. However, little is known about the role of MSI in esophageal carcinogenesis. In the present study, we conducted microsatellite assays on 41 esophageal carcinomas and also on 44 dysplasias of the esophagus with 7 separate microsatellite loci. MSI was detected in 17 cases (42%) among 41 esophageal carcinomas. MSI negative cases revealed greater lymph node metastasis, metastasis at a more advanced stage, a higher recurrence level and a poorer prognosis (statistically not significant). In the analysis of dysplasias, MSI was detected in 26 lesions (59%) among 44 lesions. Interestingly, MSI was detected in 21 lesions (78%) from the mutator phenotype dysplasias, but detected in only 5 lesions (29%) from the non-mutator phenotype cases. Although the significance of MSI in esophageal carcinoma was not clear, these results indicate that MSI occurs in the early stage of esophageal carcinogenesis.