Recent reviews described a cumulative risk of 0-38% of oral leukoplakias (OL) to transform into oral squamous cell carcinoma (OSCC). The annual transformation rate (ATR) of OL into OSCC amounted up to 6.3%. To elucidate epidemiological consequences of varying ATR. Variations of the prevalence (1-2%) and ATR (0.5-4%) of OL were used to compute the proportional incidences of OSCC developing in OL. These were compared to the actual incidences of OSCC excerpted from European cancer registries. A ratio <50% of predicted to actual incidence overall indicated a plausible ATR. With an ATR>or=3%, the predicted incidences of OSCC would exceed the reported incidences in all countries. With an ATR<or=1%, the ratio of OSCC developed in OL to OSCC overall would be always lower than 50%. Based on present data, it was concluded that the ATR of OL into OSCC should not exceed 1%.