We report the presenting neurologic and visual symptoms in 17 patients with primary thrombocythemia. Poorly localizing symptoms occurred in 14 patients: transient unsteadiness in 13, dysarthria in eight, and scintillating scotomata in four. Nine patients had focal symptoms: transient monocular blindness in two patients, transient mono- or hemiparesis in six, and both of these in one patient. The attacks all had a sudden onset, occurred sequentially rather than simultaneously, lasted for a few seconds to several minutes, and were usually associated with a dull or pulsatile headache. A causal platelet-mediated arterial thrombosis was evident in 15 patients. There were no recurrences of thrombotic or ischemic events during long-term treatment of 12 patients with aspirin and during cytostatic reduction of platelet count to normal in seven patients. We conclude that low-dose aspirin and reduction of platelet count to normal are effective, but coumarin is ineffective, in the treatment and prevention of arterial thrombosis in thrombocythemia.