METHODS The Agincourt demographic and health surveillance site in South Africa's rural Northern Province. OBJECTIVE To accurately assess the true burden of tuberculosis in a rural sub-district with a known high prevalence of human immunodeficiency virus. METHODS Data on hospital registrations of tuberculosis were combined with data from an ongoing demographic health and surveillance system to accurately describe the burden of tuberculosis in a well-defined community. Undiagnosed active cases of sputum-positive disease in the community were detected among chronic coughers identified by heads of household during a single-pass census interview. RESULTS The incidence of hospitalised tuberculosis among the permanently resident population (n = 56 566) was 212/100,000 person-years during 1999. The average point prevalence of detected tuberculosis (all forms) among patients aged over 10 years was 133/100,000, and 81/100,000 for sputum-positive pulmonary disease. This compares with a point prevalence of 16/100,000 cases of sputum-positive disease detected through active case finding. CONCLUSIONS For every nine cases of sputum positive pulmonary tuberculosis being treated at any one time, there are two cases of undiagnosed disease in the community. This study demonstrates a modest burden of undiagnosed tuberculosis among residents in a rural sub-district in South Africa.