Most cases of Mesothelioma, approximately 75%, start in the chest cavity. These types of Mesothelioma are called Pleural Mesotheliomas.  The second most common form of Mesothelioma, comprising 10% to 20% of Mesothelioma cases, begins in the abdominal cavity.  These are called Peritoneal Mesotheliomas.  Other places where Mesothelioma may form, although very rarely, is around the heart (Pericardium Mesothelioma) and around the male (Tunica Vaginalis Testis Mesothelioma) and female (Tunica Serosa Uteri Mesothelioma) reproductive organs.