RESUMEN
Intracranial arachnoid cysts are collections of clear fluid within the arachnoid membrane because of nondisjunction or duplication of the structure. They are rare, represent only 1% of intracranial masses in newborns. Etiologically, they are thought to be due to maldevelopment of the arachnoid or secondary to trauma or infection. The arachnoid cyst by ultrasonogram shows characteristic well capsulated homogeneous hypoechoic cyst. Many arachnoid cysts remain asymptomatic, however, and become symptomatic later in life to cyst growth-resulting in a compression, displacement and irritation of the surrounding structures. So, early diagnosis and treatment of these cysts are important. We present two cases of arachnoid cysts diagnosed in utero by ultrasonogram. In one case, postnatal MRI was checked and confirmed arachnoid cyst with choroid plexus cyst. And other case, postnatal brain USG was checked and diagnosed arachnoid cyst. Both children are asymptomatic, and presents normal development until now. But latter children underwent surgical treatment due to persistent huge cyst and hydrocephalus.