Eleven malignant mesotheliomas of the testicular tunica vaginalis occurred in patients aged 12 to 76 (mean, 54.1) years. Hydrocele, with or without an associated mass, or appreciation of a paratesticular mass accounted for the clinical presentation. One patient had a history of asbestos exposure. Grossly, the tumors typically presented as multiple nodules studding a hydrocele sac, frequently associated with a mass infiltrating the spermatic cord or adjacent testis. Microscopically, five tumors were epithelial and six biphasic, with the typical architectural and cytologic features of mesothelioma. Mixtures of papillary, tubular, and solid patterns predominated in the epithelial areas; interlacing fascicles of spindle cells with scanty stroma characterized the sarcomatous components. All eight of the tumors that were stained for keratin (AE1/AE3) were positive, four of five for epithelial membrane antigen, and four of five for vimentin. Seven of seven tumors were carcinoembryonic antigen negative and five of five B72.3, Leu-M1, and Ber-Ep4 negative. Follow-up ranging from 1 to 15 (mean, 4.3) years was available for seven patients. Three died of disease after 4, 4, and 3 years, and three are alive with disease 2, 2, and 15 years after diagnosis. Two of the latter three patients had extensive local recurrences, one 15 years after the diagnosis of a well-differentiated papillary mesothelioma, the other 2 years following treatment with hydrocelectomy only. One patient who has been followed for only 1 year has no evidence of disease. This series emphasizes a number of important features of testicular mesothelioma; (a) a wide age range with occasional occurrence at a young age, (b) a wide morphologic spectrum with regard to degree of differentiation, and (c) an aggressive natural history with a potential for late recurrence or metastasis of even well-differentiated tumors, suggesting the need for initial aggressive surgical treatment.