Prepubertal testicular tumors are rare, and teratoma is the second most frequent histologic type. Its typical features are those of a hard and painless scrotal mass at clinical examination, and nonhomogeneous, echoic, often with calcifications at ultrasonography. Rare but reported is the atypical presentation as a transilluminating scrotal mass, due to the presence of some internal cystic areas, detectable at ultrasonography. We report the case of an infant with a transilluminating scrotal mass, mimicking at ultrasonography and surgery a simple, fully liquid cyst, which the pathologic examination revealed to be mature cystic testicular teratoma.