Here is a large hydrocele of the testis. Such hydroceles are fairly common, though most do not reach this size. Clear fluid accumulates in a sac of tunica vaginalis lined by a serosa. It is a remnant of the embryologic processus vaginalis. It may arise in association with a variety of inflammatory and neoplastic conditions. A hydrocele must be distinguished from a true testicular mass, and transillumination may help, because the hydrocele will transilluminate but a testicular mass will be opaque.