The clinicopathological significance of cell-cycle proteins has remained unclear in oral tongue squamous cell carcinomas (OTSCC). In the current study, we evaluated several cell-cycle proteins in relation to clinicopathological parameters and disease outcome for OTSCC. A total of 123 previously untreated patients with OTSCC, who underwent surgical treatment, were enrolled. Tumor specimens were examined for expression of p21, p27, p16, p53, and p63 using immunohistochemistry, with reference to clinicopathological factors and disease outcome. It is noteworthy that differences in p21 immunoreactivity were evident between the shallow region and invasive front of tumors within the same specimens. Loss of p21 expression in invasive fronts was found to be associated with clinicopathological factors of tumor progression and poor prognosis. p21 expression in invasive fronts is a significant indicator for impact on survival. Moreover, p21 is one of the important factors that regulate the progression of malignant cells in OTSCC.