Purpose: Although obesity is a well-established risk factor for many cancers, the effect of body mass index (BMI) on oral cancer risk remains controversial. We therefore investigated the effect of BMI on oral cancer risk in a case-control study in Korea. Methods: Overall, 364 patients with oral cancer and 439 community controls were enrolled. Odds ratios (OR) and 95% confidence intervals (CI) were estimated using logistic regression models, adjusted for age, smoking status and alcohol consumption. Results: We found no overall significant evidence of an association between oral cancer risk and BMI in either gender. However, when the relationship between BMI and oral cancer risk was examined according to female age groups (<50 and ${geq}$50 years), there was a significant association between oral cancer risk and high BMI in female subjects younger than 50 years of age (OR=3.92, 95% CI 1.03~14.9, $P$ for trend=0.04), but not in older (${geq}$50 years) female subjects (OR=1.11, 95% CI 0.55~2.24, $P$ for trend=0.76). There was no significant relationship between BMI and oral cancer risk in any of the male age subgroups. Conclusion: Our study provides the first epidemiological evidence supporting an association between obesity and an increased risk of oral cancer.