We report the occurrence of unilateral cerebral hemisphere edema with subsequent cortical laminar necrosis in the setting of familial hemiplegic migraine (FHM) and permanent neurologic sequelae after resolution of an attack in 1 patient. Contemporaneous with this severe attack of FHM, the patient was found to exhibit multiple systemic and neurological symptoms referable to Erdheim-Chester disease (a rare non-Langerhans cell histiocytosis) that was confirmed by bone biopsy. This case demonstrates the severity possible with a migrainous infarction associated with FHM. The co-occurrence of two such rare entities in 1 patient suggests a possible relationship.