We investigated mucin 1(MUC1) expression in 10 vulvar condyloma acuminata(VCA), 15 vulvar intraepithelial neoplasia(VIN) and 30 vulvar squamous cell carcinomas (VSCC) using three different monoclonal antibodies(Ma695, CA15−3, and DF3), and compared the level of MUC1 expression with the clinicopathologic features of VSCC. Of the three monoclonal antibodies, Ma695 showed the strongest immunoreactivity. MUC1 expression levels were high in VSCC, low in VIN III, and undetectable in VIN I-II, VCA and normal vulvar epithelium. Patients with human papilloma virus(HPV)−negative VSCC showed higher MUC1 expression than HPV−positive patients. The MUC1 expression level gradually increased from well through moderately to poorly differentiated VSCC, and there was a statistically significant difference between moderately/poorly differentiated VSCC and well differentiated VSCC(p<0.05). These results indicate that MUC1 expression may be an effective tumor marker for VSCC, and suggest that MUC1 gene expression may be a late event in the growth of VSCC. Moreover, it appears likely that MUC1 expression correlates negatively with HPV infection. Since the appearance of MUC1 is linked to cell dedifferentiation, it may be related to the malignant transformation and progression of VSCC.