In a house-to-house survey, 36 471 tobacco chewers and smokers were selected from the rural population in three areas of India. These individuals were interviewed for their tobacco habits and examined for the presence of oral leukoplakia and other precancerous lesions, first in a baseline survey, and then annually over a 5-year period. By personal advice and via the mass media they were encouraged to give up their tobacco habits. The follow-up rate was 97%. The control cohort was provided by the first 5-year results from a 10-year follow-up study conducted earlier in the same areas with the same methodology but on different individuals without any educational intervention. In Ernakulam district (Kerala) and Srikakulam district (Andhra) substantially more people stopped their tobacco habit and reduced the frequency of tobacco use in the intervention cohort than in the control cohort; in Bhavnagar district (Gujarat) the intervention group showed only a slightly higher proportion stopping their tobacco habits and no difference in the proportion reducing them. The 5-year age-adjusted incidence rate of leukoplakia in Ernakulam district was 11.4 in the intervention group versus 47.8 among men, and 5.8 versus 33.0 among women; and for palatal lesions in Srikakulam district the corresponding figures were 59.8 versus 260.8 among men and 289.5 versus 489.5 among women. In Bhavnagar the incidence rate of leukoplakia did not differ between the cohorts. Since most oral cancers are preceded by precancerous lesions, education on tobacco habits should be a feasible and effective approach to primary prevention of oral cancer.