The Authors aim was to study the Labeling Indices (LI) of 12 subjects with oral leukoplakia, using an in vitro labeling technique with tritiated thymidine (Silvestrini-Kit, Ribbon spa, Milan). Twelve volunteers of our Dental Clinic were enrolled in this study as a control group. The kinetic parameter was investigated by counting cells from autoradiographs on histological selections of the biopsies. By a Wilcoxon test, significant differences were found between the LI of the two groups: p = 0.00003. Finally, a comparison has also been made between LI of these lesions and the LI of oral squamous cell carcinomas.