Transient monocular blindness in the patient with polycythemia vera has generally been considered to be caused by retinal arteriospasm. In the present case with complaints of frequent monocular blindness, ophthalmoscopic observation and serial photographs of the ocular fundus were made. During these acute attacks, microembolic phenomena were noticed in the retinal arteries. Laboratory examination showed abnormal platelet aggregability, increased platelet adhesiveness and a significantly decreased platelet count during attacks. It is suggested that transient monocular blindness is caused by platelet emboli. The cause of intermittent cerebral ischemic attacks in the patient with polycythemia vera is probably platelet emboli.