The case of a 15 year old girl with a rapidly progressing right sided hemiparesis is reported. The history and clinical manifestations were suggestive of either herpes simplex encephalitis or occlusion of the left middle cerebral artery. At necropsy an occlusive thrombosis of the left middle cerebral artery due to a segmental necrotizing granulomatous panarteritis was found. The cause was obscure. A plausible incriminating factor was the contraceptive pill.