Cell proliferation in oral leukoplakia has been studied by estimating the number of cells in the S phase using in vitro 3H thymidine labelling. The labelling index was found to be increased 1.77 fold in leukoplakias compared to the clinically normal control sites (P < 0.005). This rise in labelling index was found to be related to an increase in the proportion of the cells in the basal cell layer engaged in cell production. Both leukoplakia and the control specimens showed epithelial atypia, although in the latter samples this was mild yet suggestive of a possible field change in the oral epithelium of a small group of tobacco users. A significant positive correlation was found between the epithelial atypia score and the labelling index. This raises the possibility of using the labelling index as an objective prognostic marker in assessing potential malignancy in oral leukoplakia lesions.