To better define the prevalence, presentation, primary sites and survival of patients with Carcinomatous involvement of the pleura, we reviewed 96 cases of carcinoma of the pleura diagnosed by cytopathology or closed pleural biopsy at Colorado General Hospital from 1960 to 1975. Carcinomatous pleura) metastasis was present in 43/100,000 persons admitted to the hospital. The most common primary sites were lung in 32 of 96 (33 per cent), breast in 20 of 96 (20.9 per cent), ovary in nine of 96 (9.3 per cent) and stomach in seven of 96 (7.3 per cent). The incidence of pleural metastasis per type of carcinoma was lung in 32 of 459 (7.0 per cent), stomach in seven of 195 (3.6 per cent), breast in 20 of 645 (3.1 per cent) and ovary in nine of 303 (2.9 per cent). Chief presenting symptoms included dyspnea (57 per cent), cough (43 per cent) and chest pain (26 per cent); however, 22 of 96 subjects (23 per cent) were asymptomatic. Ninety-two per cent of the lung, breast and ovarian malignant effusions were ipsilateral to the primary lesion. The malignant pleural effusions were usually clear or serosanguineous exudates with a protein content of 3.7 ± 0.2 g/100 ml (mean ± standard error of the mean (SEM); lactic dehydrogenase 134 ± 15 lU/liter; glucose 120 ± 13 mg/100 ml, white blood cell count 2,250 ± 400/mm3. Mean survival was 3.1 ± 0.5 months (after diagnosis of pleural metastasis) with 54 per cent mortality within one month and 84 per cent mortality by six months. A malignant pleural effusion provided the basis for the first diagnosis of cancer in 44 of 96 patients (46 per cent). Pleural involvement, although often asymptomatic, is an ominous finding usually representing widespread metastases. Lung, breast, ovary and stomach were the most frequent primary sites, with carcinoma of the lung being the most common to involve the pleura; however, its incidence was relatively low. A malignant pleural effusion frequently represents the first evidence of cancer.