Summary The level of p27 expression decreases during tumor development and progression. Loss of p27 protein provides independent prognostic information in breast, prostate, colon, stomach and lung carcinomas. We generated a new polyclonal antibody against p27 and carried out immunohistochemical analysis of p27 expression in 61 specimens of esophageal squamous cell carcinoma, 10 carcinoma in situ specimens, and 5 squamous epithelial dysplasia specimens of the esophagus. We examined the correlation of p27 expression with various clinicopathologic features and prognosis. In squamous epithelial dysplasia, the p27-positive cells were located in the superficial normal-appearing cells, not in the atypical cells. In carcinoma in situ , the expression of p27 was markedly lower than in normal esophageal epithelium. In advanced squamous cell carcinomas, high p27 correlated significantly with the degree of differentiation (p = 0.0002) and the depth of tumor invasion (p = 0.001) was statistically significant. The high p27 expression group had a better prognosis than did the low or negative p27 expression groups, but these differences were not statistically significant. These observations may imply that a decrease in p27 expression occurs early on in the carcinogenesis of esophageal carcinoma. In addition, p27 expression may correlate with the histologic differentiation of squamous cell carcinoma of the esophagus.