A newborn girl was admitted suffering from convulsive seizures in both legs. A computed tomography (CT) scan revealed bilateral symmetrical middle cranial fossa arachnoid cysts and intraventricular hemorrhage. In the course of a year, only the cyst on the right side became gradually larger; the cyst on the left side was reduced in size. At the age of 1 year and 2 months, her intellectual development was normal, but she could not stand by herself. On metrizamide-enhanced CT cisternography, the cyst on the right side was of a noncommunicating type. Membranectomy of the right-sided cyst was performed and the cyst proved to be of the intraarachnoid type. The postoperative course was uneventful and a CT scan, taken 10 months after the operation, revealed no abnormality on either side. The intellectual and motor development was normal, and no abnormality was observed by electroencephalography. This may be the first report of arachnoid cysts found in a neonate. It gives us clues for understanding the etiology of this disease, which has not been fully clarified.