A 56-year-old man presented with a painless swelling of the left scrotum. Cytologic examination of blood stained hydrocele fluid suggested malignancy. Left high orchiectomy was performed under the suspicion of malignant tumor of the tunica vaginalis testis. The final pathologic report revealed malignant mesothelioma of the tunica vaginalis testis. There is no evidence of recurrence after 114 months followup. It is important to perform en bloc resection for this disease to prevent recurrence.