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A large-scale epidemiological study of oral cancer and precancer was initiated in 1966 in several regions of India. Phase 1 of the study consisted of a cross-section survey to determine the prevalence of cancer and precancerous lesions, while phase 2 was a 10-year follow-up survey to determine the incidence and natural history of oral precancer. Following these preliminary investigations, an intervention study was started to try to persuade subjects to give up tobacco, and to measure the subsequent changes in the incidence and regression rate of oral precancer. In each of the three study areas, 12 000 adult tobacco users were selected, examined, and interviewed in a house-to-house survey. Information on the ill effects of tobacco use was given through individual discussions with a social scientist, film shows, and posters. The population was reviewed after one year and the proportions of subjects who had discontinued tobacco use were found to be 2% in Ernakulam, 1% in Bhavnagar, and 5% in Srikakulam. Additionally, between 1% and 16% had reduced their tobacco use. In Bhavnagar and Ernakulam the regression rate of leukoplakia was significantly higher among those who had stopped or reduced their tobacco consumption. In Srikakulam there was a highly significant correlation between reduction in tobacco consumption and regression of palatal lesions.