It is extremely rare to see eosinophilic granuloma of the central nervous system without osseous or multisystemic involvement. Only two adult cases with isolated cranial nerve involvement have been reported to date. This report describes an adult patient with isolated eosinophilic granuloma of the optic chiasm. We suggest that eosinophilic granuloma should be included in the differential diagnosis of tumors involving the cranial nerves, and particularly the optic chiasm.