Mesotheliomas with clear cell morphology are rare and only a few individual case reports have been documented in the literature. The author reports a series of 20 epithelioid mesotheliomas with clear features, 17 of which originated in the pleura and 3 in the peritoneum. Eighteen of the patients were men and 2 were women. Twelve patients had a history of asbestos exposure. Electron microscopy and special histochemical stains demonstrated that the cytoplasmic clearing seen in hematoxylin and eosin-stained sections resulted from multiple factors that can occur either singly or in combination. The most frequent cause of the cytoplasmic clearing was the accumulation of large amounts of intracytoplasmic glycogen. Another but somewhat less common factor was the accumulation of large amounts of lipid, which occurred alone or with glycogen. Other less common causes were marked mitochondrial swelling, the presence of numerous intracytoplasmic vesicles, and a large number of intracytoplasmic lumens. The value of immunohistochemistry in helping to distinguish epithelioid mesotheliomas from some carcinomas with clear cell morphology is emphasized. In addition, it was determined that because electron microscopy was decisive in establishing the cause of the cytoplasmic clearing in most of the cases, tissue for electron microscopy should routinely be procured for ultrastructural studies.