When a patient presents with new pleural effusion, the diagnosis of tuberculous (TB) pleuritis should be considered. The patient is at risk for developing pulmonary or extrapulmonary TB if the diagnosis is not made. Between 3% and 25% of patients with TB will have TB pleuritis. The incidence of TB pleuritis is higher in patients who are human immunodeficiency virus (HIV)-positive. Pleural fluid is an exudate that usually has a predominance of lymphocytes. The easiest way to diagnose TB pleuritis in a patient with lymphocytic pleural effusion is to demonstrate a pleural fluid adenosine deaminase level above 40 IU/L. The treatment for TB pleuritis is the same as that for pulmonary TB. Tuberculous empyema is a rare occurrence, and the treatment is difficult. KEY WORDS:Tuberculosis, pleural effusions, adenosine deaminase, gamma interferon, pleural biopsy, empyema INTRODUCTION Tuberculosis (TB) remains a major public health problem in developing countries. Most patients with TB have pulmonary TB; however, 25% of adults can present with extrapulmonary TB, which mainly involves the lymph nodes and pleura but can affect any organ system, including the central nervous system (CNS), cardiovascular system, and gastrointestinal (GI) tract [1]. The most common form of extrapulmonary TB is infection of the pleura. In a patient who presents with undiagnosed pleural effusion, the diagnosis of TB pleuritis should be considered. Pleural effusion, as an isolated manifestation of TB, may be selflimited and of little immediate concern, but untreated, it can lead to serious disease many years later. Pathogenesis and Pathologic FeaturesTuberculous pleural effusion may occur in the absence of radiologically apparent TB [2]. The effusion may result as a sequela to a primary infection from 6-12 weeks earlier, or it may represent the reactivation of TB [2]. In developed countries, TB pleural effusions present more often in older patients. The median age in one North American study was 56, suggesting reactivation of the disease [3]. However, a study from 2004 in San Francisco showed that pleural TB cases were 2 times more likely to be clustered (when assessed by genotyping of the mycobacterial organisms) than pulmonary TB and 3 times more likely to be clustered than nonrespiratory TB cases [4]. The observations made in the San Francisco study suggest that the majority of patients has a postprimary infection [4]. This was not confirmed in studies performed in both Houston [5] and Baltimore [6] or in a study in sub-Saharan Africa, in which the prevalence of pleural TB was 63.2% in patients with a primary TB infection [7].The pathogenesis of TB pleural effusion is thought to be related to the rupture of a subpleural caseous focus in the lung into the pleural space [8]. The basis for this was the observation that a caseous TB focus could be demonstrated in the lung, contiguous with the diseased pleural, in 12 to 15 patients with TB pleuritis [9]. The 3 other patients in this study had parenchymal disease but did not have caseous foci adjacent...