Oral leukoplakia, oral submucous fibrosis and erythroplakia are 3 major types of oral premalignant lesions. Multiple oral premalignant lesions may possibly develop due to field cancerization, where carcinogenic exposures can cause simultaneous genetic defects to the upper aerodigestive tract epithelium, putting the epithelium at high risk for development of premalignant lesions at different stages of carcinogenesis. There have been no epidemiological studies on risk or protective factors of the disease. A case-control study was conducted with data from the baseline screening of a randomized oral cancer screening trial in Kerala, India. A total of 115 subjects with multiple oral premalignant lesions (8-10% of oral premalignant lesions in our case series) were included: 64 subjects with oral leukoplakia and oral submucous fibrosis, 19 subjects with oral leukoplakia and erythroplakia, 22 subjects with oral submucous fibrosis and erythroplakia and 10 subjects with all 3 lesions. Individuals without oral lesions were considered controls (n=47,773). The odds ratio (OR) for ever tobacco chewers was 37.8 (95% confidence interval (CI)=16.2-88.1) when adjusted for age, sex, education, BMI, smoking, drinking and fruit/vegetable intake. Dose-response relationships were seen for the frequency (p<0.0001) and duration of tobacco chewing (p<0.0001) with the risk of multiple oral premalignant lesions. Whereas alcohol drinking may possibly be a risk factor for multiple oral premalignant lesions, smoking was not associated with the risk of multiple oral premalignant lesions (OR=0.9, 95%CI=0.5-1.7). The results suggest that tobacco chewing was the most important risk factor for multiple oral premalignant lesions and may be a major source of field cancerization on the oral epithelium in the Indian population.