In a consecutive series of 53 cases of oral leucoplakia, smoking history, oro-dental status and arylhydrocarbon-hydroxylase (AHH) inducibility were assessed. There was an increased association of leucoplakia with smoking, particularly in the male cases, but no remarkable findings as to dental status. Overall, there was a slightly higher frequency of cases with high AHH inducibility than expected from a normal control population. This difference was confined to the smokers, where there was a statistically highly significant over-representation of high AHH inducibility and under-representation of low AHH inducibility. The figures in the non-smoking patients were close to the expected.