The cytologic, histologic and ultrastructural features of a malignant carcinomatous mesothelioma of the tunica vaginalis testis in a 30-year old patient are described. This is the first such case with preoperative diagnosis by cytologic examination of hydrocele fluid and the second with documented history of exposure to asbestos. The identity of the tumor was confirmed by a positive immunoperoxidase reaction directed against mesothelial cells. A review of the previously described ten cases is presented.