In 120 patients with atherosclerosis, complicated in 43 patients by a haemorrhagic, in 47 patients by an ischaemic, and in 30 patients by a transient cerebral insult, phase analysis of platelet aggregation was performed by the turbidimetric method according to Born with graphic recording according to O'Brien. An increase in the platelet activity was found in ischaemic insult, manifesting itself by the occurrence of spontaneous aggregationin 60% of the cases, an acceleration of ADP-induced aggregation, and the second aggregation phase in all patients examined. A direct correlation was revealed between the secondary aggregation and the intensity of spontaneous and of ADP-induced aggregation, and the possibility of a transformation of the spontaneous into the secondary aggregation of platelets was demonstrated. Haemorrhagic insults were characterized by the absence of spontaneous and secondary aggregation and by the suppression of ADP-induced aggregation. In a transient insult, the mean values of the aggregatogram items did differ from normal. In vitro, the role of increased permeability of platelet membranes in the mechanism triggering off spontaneous aggregation and the second phase of ADP-induced aggregation was documented.