Proliferation was studied in 18 patients with benign oral leukoplakias of the buccal mucosa by in vitro labelling with 3H-TdR. Identical investigations were performed in clinically normal mucosa of the same patients. Significant differences were found between the LI of both groups. On the other hand the total of labelled cells per 100 micron length of the basal membrane was approximately equal in leukoplakias and normal mucosa. In the majority of cases the LI was increased in leukoplakic regions whereas the number of labelled suprabasal cells was lowered compared to normal. These findings are supposed to be a consequence of shifting of epithelial keratinization to the lower stratum spinosum. From that results an increased proliferation in the basal cell layer.