Plasma fibrinogen levels and platelet counts were evaluated in 30 patients with acute ischemic cerebral infarction (CI) three and nine days after the onset of symptoms. Hyperfibrinogenemia (379.4 +/- 80.3 vs 327.3 +/- 48.3 mg/dL of controls, p < 0.005), a reduction of platelet count (207.133 +/- 48.388 vs 288.375 +/- 61.373 x 10(9)/L of controls, p < 0.001), and an inverse correlation (r = -0.41, p < 0.05) between the two parameters were observed on day three. On day nine, platelet counts normalized while plasma fibrinogen levels slightly increased; the inverse correlation no longer occurred. The results suggest that in the earliest phase of stroke plasma fibrinogen levels may condition the extent of platelet accumulation or consumption in the ischemic area. This confirms in vivo that the platelet aggregation process is strictly dependent on fibrinogen concentration.