⇓Malignant pleural effusions are a common clinical problem in patients with neoplastic disease. In one post mortem series, malignant effusions were found in 15% of patients who died with malignancies 1. Although there have been no epidemiological studies, the annual incidence of malignant pleural effusions in the United States is estimated to be >150,000 cases (table 1⇓) 2–17. Malignant pleural effusion is also one of the leading causes of exudative effusion; studies have demonstrated that 42–77% of exudative effusions are secondary to malignancy 18, 19. Fig. 1.— Malignant pleural effusions: sensitivity (%) of different biopsy methods (cytological and histological results combined). Presented is a prospective simultaneous comparison (n=208). Fig. 2.— Diagnostic sensitivity of cytology (□) and medical thoracoscopy (![Graphic][1] ) in malignant pleural effusions. n-numbers are as follows: lung cancer, 67; non-lung primary, 154; mesothelioma, 66; total, 287. Fig. 3.— View this table: Table 1— Incidence of malignant pleural effusions (MPEs) Nearly all neoplasms have been reported to involve the pleura. In most studies, however, lung carcinoma has been the most common neoplasm, accounting for approximately one-third of all malignant effusions. Breast carcinoma is the second most common. Lymphomas, including both Hodgkin's disease and non-Hodgkin's lymphoma, are also an important cause of malignant pleural effusions. Tumours less commonly associated with malignant pleural effusions include ovarian and gastrointestinal carcinomas. In 5–10% of malignant effusions, no primary tumour is identified 12, 13. The incidence of mesothelioma varies according to the geographical location. Post mortem studies suggest that most pleural metastases arise from tumour emboli to the visceral pleural surface, with secondary seeding to the parietal pleura 1, 20. Other possible mechanisms include direct tumour invasion (in lung cancers, chest wall neoplasms, and breast carcinoma), haematogenous spread to parietal pleura, and lymphatic involvement. A malignant tumour can cause a pleural effusion, both directly and indirectly. … [1]: F2/embed/inline-graphic-1.gif