Mesothelioma is a disease in which cells of the mesothelium become abnormal and divide without control or order. Asbestos exposure is responsible for most cases of malignant mesothelioma. In about two thirds of cases, an asbestos-exposure history is present. The risk of developing mesothelioma is directly related to how much asbestos exposure a person has had and for how long. Many studies of worker groups with heavy asbestos exposures have found increased risks of mesothelioma. However, mesothelioma has a long latency period or period to develop, and heavt asbestos exposure has been traced 20 and 30 years before the patient developed the disease.