Tumor thickness is a relatively new prognostic factor that has been investigated for lower lip cancer. This study was performed in 27 patients, 13 of whom had histopathologically confirmed cervical metastasis, to investigate whether tumor thickness could be used as a predictor of cervical lymph node metastasis. The mean tumor thickness of those cases with neck metastasis was 5.60 mm (SD 2.24), and the mean thickness of cases without neck metastasis was 3.79 mm (SD = 1.68). The difference between tumor thicknesses of both groups was found to be statistically significant (P < 0.05). The tumor thickness of 5 mm was determined as a cutoff point, above which the cervical lymph node metastasis rate was significantly increased. As a conclusion, tumor thickness is an objective histopathologic factor that is easily reproducible; it significantly influences cervical lymph node metastasis in lower lip cancer, and it may be used in the assessment of prognosis.