Approximately half of all patients with metastatic cancer develop malignant pleural effusions. Because the patients are already terminally ill, these effusions can present significant diagnostic and therapeutic challenges. Symptoms are either present at the time of diagnosis or develop subsequently in virtually all cases. The diagnosis is based on chest radiography followed by thoracentesis or thoracoscopy. Most malignant effusions are exudative and about one third are bloody. Cytology is positive for cancer cells in the initial pleural fluid specimens from 60% of patients who are ultimately shown to have malignant effusions. The remaining 40% require a repeat thoracentesis, pleural biopsy, thoracoscopy, or multiple procedures to prove the presence of cancer. Because the average life expectancy of a patient with a malignant pleural effusion is about 6 months, it is important to obtain a diagnosis expeditiously and formulate a treatment plan that optimizes quality of life. Tube thoracostomy with chemical pleurodesis using doxycycline or bleomycin is the mainstay of current treatment and is about 85% effective.