Mesothelioma is a cancer of the mesothelium, the protective sac that covers and protects most internal organs of the body. The mesothelium has two layers, one, which covers the organ, and a second, which forms a sac around it. The mesothelium has different names depending on the location in the body. The pleura is the mesothelial tissue surrounding the lungs and lining the chest cavity. The peritoneum covers most of the organs in the abdominal cavity (where abdominal mesothelioma occurs), while the pericardium covers and protects the heart.

The mesothelium produces a lubricating fluid that allows organs within it to move and glide easily. For example, the fluid allows the lungs to breathe and the heart to expand and contract.

Like other forms of cancer, mesothelioma occurs when cells become abnormal and divide or grow out of control. In mesothelioma, the helpful lubricating fluid is over-produced. This excess fluid encases the organs with a thick layer of tumor tissue, described as similar to a rind type of layer. In advanced cases of mesothelioma, cells metastasize, or grow and invade other organs and spread to other areas of the body.

The majority of individuals suffering from mesothelioma have cancer in the lining of the lung. Sometimes, mesothelioma occurs in the lining of the abdominal cavity (peritoneal mesothelioma) or in the lining of the heart (pericardial mesothelioma).