Cyclin B1 plays an important role in regulating the G2-M transition of the cell cycle. In this study, we evaluated the immunohistochemical expression of cyclin B1 in a series of 40 oral tongue squamous cell carcinomas and analyzed the findings in relation to both clinicopathological variables and the Ki-67 labeling index. In normal squamous epithelium, cyclin B1 was localized in the nucleus and was expressed only in several cells of the basal and parabasal layers. By contrast, in dysplastic epithelium, cyclin B1 was localized in the cytoplasm and nuclei, and the numbers of cyclin B1 positive cells increased compared with normal epithelium. In tumor tissues, cyclin B1 expression was observed mainly in the cytoplasm, and the cyclin B1 labeling index ranged from 2 to 21%, with a mean of 11%. Cyclin B1 overexpression was positively correlated with occult cervical lymph node metastases and the number of mitotic cells. In addition, there was a positive relationship between labeling indices of cyclin B1 and Ki-67. These findings indicate that cyclin B1 could be a useful prognostic marker of occult cervical lymph node metastases in tongue carcinoma.