Platelet aggregation in the post-acute phase of 48 patients with cerebral thrombosis was measured to see if any specific type of cerebral infarction is associated with enhanced platelet aggregation. All patients were examined with cerebral angiography and computed tomography (CT). Stenotic lesions in major cranial arteries were analyzed by measuring the apparent diameter. Severe stenosis was defined as 75 per cent constriction or more. Enhanced aggregation of platelets (secondary aggregation at 1 microM ADP or less) was present in 5 of 25 patients (20%) who had severe vessel stenosis or occlusion. CT examination frequently revealed both cortical and deep involvement. On the other hand, 13 of 23 patients (57%) with less stenotic lesions showed enhanced aggregation and that was statistically significant (p less than 0.05). Many patients of this group had persistent hypertension and small deep infarctions. Platelet aggregation was also measured in 20 hypertensive control subjects without stroke. Four of them (20%) showed enhanced aggregation. These findings suggest that a combination of enhanced platelet aggregation and hypertension increases the risk of small deep infarctions accompanied by mild stenotic changes of the major cranial arteries.