Detection of TBSA was attempted in pleural aspirates of 74 patients with tuberculous and 44 patients with nontuberculous pleural effusion by gas chromatography/mass spectrometry with selected ion monitoring. The results were disappointing with a test sensitivity of 67.6 percent and a specificity of 52.3 percent. In contrast, histologic examination of pleural biopsies gave a diagnostic sensitivity of 71.0 percent. Pleural biopsy remains a better investigational procedure for the diagnosis of tuberculous pleural effusion.