The etiology and mechanism of expansion of primary intracranial arachnoid cysts have been much debated. A rare case of an 8-month-old boy is reported, in which postnatal development and enlargement of a middle cranial fossa arachnoid cyst was detected on follow-up CT scans. Based on intraoperative and histological findings, the cyst was found to be intra-arachnoid. The wall was excised completely, and the lobe adjacent to the cyst appeared normal apart from signs of atrophy. Histological study of the excised cyst revealed a common arachnoid membrane with neither ependymal nor inflammatory cells; the cyst fluid was similar to CSF. The etiology of the lesion remains unclear, but it was considered that the expansion of the cyst might have occurred through a ball-valve mechanism of the membrane in communication with the general subarachnoid space.