We investigated the prevalence of, and risk factors for, oral potentially malignant disorders (OPMDs) in rural Sri Lanka. A cross-sectional community-based study was conducted by interview and oral examination of 1029 subjects aged over 30 years. A community-based nested case-control study then took those with OPMDs as 'cases', "controls" being those with no oral abnormalities at time of initial screening. The prevalence of OPMD was 11.3% (95% CI: 9.4-13.2), after weighting for place of residence and gender. Risk factors were betel-quid (BQ) chewing daily [OR=10.6 (95% CI: 3.6-31.0)] and alcohol drinking daily or weekly [OR=3.55 (1.6-8.0)]. A significant dose-response relationship existed for BQ chewing. Smoking did not emerge when adjusted for covariates. A synergistic effect of chewing and alcohol consumption existed. The attributable risk (AR) of daily BQ chewing was 90.6%, the population AR 84%. This study demonstrates high prevalence of OPMD, betel-quid chewing with or without tobacco being the major risk factor.