Patients presenting with pleural effusion of undetermined aetiology were prospectively enrolled, and an enzyme-linked immunospot (ELISPOT) assay on pleural fluid and peripheral blood was performed. Forty patients were studied, including 19 with culture- or biopsy-confirmed (n = 15) or clinically compatible (n = 4) tuberculous pleurisy, and 21 with pleural effusions due to non-tuberculous causes. The sensitivity, specificity and positive and negative predictive values of the assay were 94.7%, 85.7%, 85.7% and 94.7%, respectively, on pleural fluid, and 77.8%, 90.5%, 87.5% and 82.6%, respectively, on blood. Antigen-specific, interferon-gamma-secreting T-cells were concentrated eight to ten times in pleural fluid as compared with blood. Among the seven patients not suitable for pleural biopsy and three patients whose biopsy results were non-diagnostic, nine had positive ELISPOT result with pleural fluid. The ELISPOT assay for interferon-gamma can accurately diagnose tuberculous pleurisy and is helpful for patients not suitable for pleural biopsy and those whose biopsy results are non-diagnostic.