The author's aim was to evaluate the cell proliferation of oral lichen planus, by an in vitro labelling technique using tritiated thymidine (TLI). For this purpose six patients with oral lichen planus and six control volunteers with oral mucosa clinically sound were studied. The cell kinetic parameter was investigated by counting cells from autoradiographs on histological sections of the biopsies. Significant differences were found between the basal TLI of the two group (p = 0.002) but no between the epithelial TLI (Wilcoxon test).