Platelet aggregability was measured using the screen filtration pressure (SFP) method in SO elderly healthy people, 93 persons with essential hypertension, 166 patients with cerebral thrombosis at the recovery stage (more than two months after onset), and 74 patients with cerebral hemorrhage at the recovery stage. SFP by 3 liM ADP in the healthy persons, the hypertensive patients, and the patients in the recovery stages of hemorrhage and thrombosis were 148.7 ± 53.5, 176.2 ± 74.4, 189.8 ± 58.3 and 206.3 ± 58.9 mm Hg, respectively. The differences of the SFP between the healthy and each of the diseased groups were statistically significant (P < 0.01 to 0.05). Meanwhile, SFP of nine patients with cerebral thrombosis and 18 patients with hemorrhage was measured during their time course of the diseases from the onset to 180 days. SFP in the acute stage of thrombosis showed an increase and a gradual decrease during the time course, while SFP in the acute stage of hemorrhage showed the opposite - a decrease and a gradual increase. A statistically significant difference was observed between both groups within 30 days from the onset (P < 0.01). Screen filtration pressure in the acute stage of hemorrhage showed 95.2 ± 17.7 mm Hg in nine survival cases and 194.0 ± 96.2 mm Hg in nine deaths within ten days from the onset. The difference was statistically significant (P < 0.01). Such results suggest a role of platelets in cerebral thrombosis and hemorrhage and a usefulness in differential diagnosis of both diseases.