Mesothelioma: A malignant tumor of the mesothelium, the thin lining of the surface of the body cavities and the organs that are contained within them. Most mesotheliomas begin as one or more nodules that progressively grow to form a solid coating of tumor surrounding the lung, abdominal organs, or heart. Mesothelioma occurs most commonly in the chest cavity and is associated with exposure to asbestos in up to 90 percent of cases. The risk of mesothelioma increases with the intensity and duration of exposure to asbestos. Family members and others living with asbestos workers may also have an increased risk of developing mesothelioma and possibly other asbestos-related diseases. This risk may be the result of exposure to asbestos dust brought home on the clothing and hair of asbestos workers. Mesothelioma is currently difficult to treat in most cases, and carries a poor prognosis.