Primary mediastinal mesotheliomas are rare tumours. The mesothelial lining cells of the the pericardium are suggested as the most probable cells of origin. Most of these tumours appear either as a diffuse or nodular thickening of the pericardium that encase and even invade the heart. Localized mediastinal mesotheliomas are distinctly uncommon. We report the imaging findings of a solitary malignant mediastinal mesotheliorna that presented mainly as a cystic anterior mediastinal mass. On chest radiography, the tumour appeared as a right paramediastinal soft tissue mass located adjacent to the right middle and lower lobes. On CT, a large, well-circumscribed, right anterior mediastinal mass with a central zone of fluid attenuation was observed. This mass had a thin, smooth wall of uniform thickness as well as a small component that demonstrated soft tissue attenuation. There was no plane of separation between the tumour and aorta/superior vena cava. At surgery the tumour could be dissected easily free from the pericardium and great vessels and it was removed totally. Histopathological examination of the tumour revealed a malignant epitheloid mesothelioma.