The Karunagapally cohort in Kerala, India was established in the 1990s. The present study examined oral cancer risk among 66,277 men aged 30-84 years in the cohort, using Poisson regression analysis of grouped data, stratified on attained age, calendar time, education, and family income. By the end of 2005, 160 oral cancer cases were identified by the Karunagapally Cancer Registry. Tobacco chewing increased oral cancer risk (P < 0.001). Particularly increased was the risk of cancers of the gum and mouth (relative risk [RR] = 4.7; 95% confidence interval [CI] = 2.8-7.9), which increased with higher daily frequencies (P < 0.001) and longer duration (P < 0.001) of tobacco chewing. Alcohol drinking was not significantly related to oral cancer risk regardless of tobacco chewing. Bidi smoking significantly increased oral cancer risk (RR = 2.6; 95%CI = 1.4-4.9) only among men without tobacco chewing habits. The risk increased with higher daily consumption (P < 0.001), longer duration (P = 0.001), and younger age at start of bidi smoking (P = 0.007). In location-specific analysis, bidi smoking was significantly associated with cancer of the gum and mouth (RR = 3.6; 95%CI = 1.1-12.1), and its risk significantly increased with larger daily consumption of bidis (P = 0.013) and younger age at the start of smoking (P = 0.044). Tongue cancer risk was significantly increased among men who smoked bidis for 30 years or longer, and men started bidi smoking at 18 years old or younger. The present study is the first cohort study showing that tobacco chewing increases cancers of the gum and mouth among men keeping chewing tobacco in the cheek, and that bidi smoking strongly increased oral cancer risk among men without a tobacco chewing habit.