BACKGROUND We investigated the incidence of tuberculous pleurisy among patients with adenosine deaminase (ADA) levels of 50 IU/L or less in a pleural effusion, and without a previous diagnosis of carcinomatous pleurisy or Mycobacterium tuberculosis. METHODS Subjects were selected from patients who had undergone pleural biopsy by thoracoscopy at National Hospital Organization Tokyo Hospital from January 1995 to November 2004, and who had ADA levels of less than 50 IU/L in pleural fluid obtained preoperatively by thoracentesis. In all subjects, smear, culture, and polymerase chain reaction for Mycobacterium tuberculosis were negative. RESULTS Of 138 patients who underwent thoracoscopic pleural biopsy, a total of 50 had effusions with ADA levels of less than 50 IU/L. Six (12%) of these patients were diagnosed with tuberculous pleurisy after biopsy. Three patients with an effusion ADA level of 35 IU/L or less were diagnosed with tuberculous pleurisy. CONCLUSIONS Occult tuberculous pleurisy is significantly common in patients with pleural effusion ADA levels of 50 IU/L or less and who may otherwise be diagnosed with nonspecific pleurisy.