There is a need for objective methods of assessment of oral epithelial precancerous lesions and reliable markers for the prediction of malignant change in these lesions. Cell proliferation was examined in 20 dysplastic lesions from the tongue and floor of mouth using bromodeoxyuridine (BrdU) and Ki-67, and a histological compartment analysis was performed. Half of a fresh biopsy from each case was incubated in BrdU for 15 min, the other half was routinely processed and used for Ki-67 analysis. Sections from each block were immunohisto chemically stained with antibodies against BrdU and Ki-67. Dysplasia was graded according to the method of Smith & Pindborg. The BrdU labelling index (LI) and the growth fraction (GF), assessed by the use of Ki-67, was quantified and expressed as units per millimetre basement membrane length (BL) and per 100 total nucleated cells (TNC). The mean LI/TNC was 10.87 (SD 3.65) and the mean LI/BL was 51.55 (SD 20.75). The mean GF/TNC was 26.66 (SD 17.78) and GF/BL was 157.07 (SD 125.84). The mean epithelial thickness was 229.09 microm (SD 104.73). The LI/BL correlated with the atypia score and with the GF/BL. The progenitor compartment sizes also correlated with the atypia scores. The BrdU labelling index provides a further objective measurement of oral epithelial dysplasia and the progenitor compartments were large, implying that basal cell hyperplasia is a significant component of the dysplasia.