Mitral valve prolapse (MVP) is a predisposing factor for cerebral ischemia, especially in young adults. Cerebral embolization of intracardiac thrombi is the probable mechanism in many cases. Platelets play a key role in the development of thrombi. We found that platelet factor 4, a marker protein of platelet activation, was elevated in 12 of 33 MVP patients (36%) without a history of stroke. This finding indicates that platelets are frequently activated in asymptomatic MVP patients and may allow identification of a subgroup of MVP patients with activated platelets who are at increased risk for emboli.