Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is an inherited disease of small arteries caused by mutations in the Notch3 gene. Complex migrainous episodes, such as acute confusional migraine, status migrainosus with persisting aura, and "CADASIL coma" have been described in patients with CADASIL. However, there are few descriptions of effective treatment of such episodes. We describe a 44-year-old male with CADASIL, who presented with sudden-onset aphasia and decreased responsiveness after prolonged, severe migraine attack. Subsequently, the patient had two generalized seizures. A subtle status epilepticus was suspected because of drowsiness and seizures, and intravenous sodium valproate medication was initiated. EEG recording showed left hemispheric attenuation but no spike discharges, thus not confirming epileptic mechanism. The clinical status of the patient improved markedly after the initiation of valproate. The patient started speaking again; drowsiness and headache subsided. In repeated EEG recording, the left hemispheric attenuation disappeared. Diffusion weighted MR imaging showed no signs of recent ischemic events. The patient recovered fully from the episode with no further seizures. We suggest that CADASIL patients with acute complex migrainous episodes may benefit from intravenous sodium valproate.