A case of a middle fossa arachnoid cyst which spontaneously disappeared is reported. A 16 year old boy visited our out-patient clinic complaining of headache after a blunt head trauma by a motorcycle accident. The neurological examination was negative except for mild neck pain on the flexion and extension of neck. Plain x-ray of skull and neck were negative. CT scan demonstrated an arachnoid cyst in the left Sylvian fissure. It was planned to follow up the patient by CT scan. After 10 months, he was examined by CT scan, which revealed the spontaneous disappearance of the arachnoid cyst and some dilatation of the anterior horn of the lateral ventricle. The patient did not have any additional trauma or infection during the interval between two examinations. Spontaneous disappearance of arachnoid cyst was extremely rare as only 4 cases were reported previously. The basic mechanism of this natural cure was considered to be rupture of outer or inner membrane of arachnoid cyst which would facilitate absorption of its content.