Fisher is credited with describing the clinical syndrome of the transient ischemic attack ("mini-stroke").  Fisher proved, by a series of pathological studies, the relationship between stroke and the formation of blood clots in the heart in patients with atrial fibrillation. He also showed the relationship between stroke and carotid artery stenosis, which made preventive surgery possible and greatly reduced the incidence of subsequent strokes.  He was a founder of Massachusetts General Hospital Stroke Service.  He contributed greatly towards the current use of anticoagulants for stroke prevention in atrial fibrillation .