Tuberculostearic acid (TBSA) (10-methyloctadecanoic acid) is a structural component of mycobacteria and is not normally present in human tissues. The detection of this fatty acid in various clinical specimens, such as sputum, bronchial aspirate, pleural fluid and cerebrospinal fluid, is a rapid, sensitive and specific test for the diagnosis of tuberculosis. As rapid diagnostic methods are also needed in tuberculous pleurisy, this study was planned on 21 patients with pleurisy. The methods of gas chromatography, mass spectrophotometry combined with selected ion monitoring were used. The patients were divided into two groups: 11 with tuberculous effusion and 10 with nontuberculous patients. In the tuberculous group, TBSA was detected in 6 of the 11 patients, whereas in the nontuberculous group it was detected in 12 of the 10 patients. The sensitivity of the test was 54%, the specificity was 80%, the positive predictive value was 75%, the negative predictive value was 61%, and the efficacy was 66%. Therefore, tuberculostearic acid cannot be considered to be more useful than conventional methods in the diagnosis of tuberculous effusions, due to low sensitivity and high cost.