In a prospective epidemiologic house-to-house survey of a random sample in the district of Ernakulam in Kerala State, the annual incidence rate of leukoplakia per 1,000 adults was found to be 2.1 for males and 1.5 for females. The rate was highest in the mixed tobacco habits group and lowest (0) in the no habits group. During the same period, oral cancer developed only among the individuals, who had a history of a previously diagnosed oral lesion. Malignant transformation was significantly higher among the speckled leukoplakia cases. The rate of malignant transformation was also highest among leukoplakias associated with tobacco chewing habits. These results suggest that leukoplakias associated with different tobacco habits may have a different natural history.