Severe impulsiveness in the absence of apparent neurological signs has rarely been reported as a clinical presentation of multiple sclerosis (MS). An 11-year-old female developed progressive and sustained personality disturbances including disinhibition, hypersexuality, drug abuse, aggressiveness and suicide attempts, without neurological signs. She was given several unsuccessful psychopharmacological and psychotherapeutic interventions. At age 21, a diagnosis of MS was made, confirmed by imaging, laboratory and neurophysiological studies. Although unusual, MS may produce pure neurobehavioral disturbances. In the present case, widespread demyelinization produced a complex behavioral disorder, with features compatible with orbitofrontal and Klüver-Bucy syndromes.