A 9-year-old boy was admitted with a right parietal local swelling and tenderness. Plain X-rays and computerized tomography scan showed an osteolytic lesion at the right parietal cranium and a giant arachnoid cyst at the left temporoparietal region. The lesion was removed, histological examination revealed an eosinophilic granuloma. Immunohistochemical analysis of the specimen were positive for the S 100 protein and CD la cell surface molecule. No further surgery was performed for the arachnoid cyst, as each pathologic condition is considered apart. Also to our knowledge, neither co-existance of eosinophilic granuloma and arachnoid cyst nor eosiniphilic granulome of the skull secondary to aracnoid cyst have not been described previously.