Mesothelial cells normally line the body cavities, including the pleura, peritoneum, pericardium, and testis. Malignancies involving mesothelial cells in these body cavities are known as malignant mesothelioma, which may be localized or diffuse. Diagnosis is difficult because the results from fluid analysis of the effusion from the tumor are not usually diagnostic. Most, but not all, pleural malignant mesothelioma is associated with asbestos exposure. Of patients with pleural malignant mesothelioma, 77% have been exposed to asbestos in the past. Mesothelioma is more common in males than in females and it occurs in the fifth and seventh decade of life. Most of malignant mesothelioma occur in the pleura (90% of the time).