AIM:To evaluate the effect of mannitol at different doses (125 ml and 250 ml) by transcranial Doppler (TCD) sonography in patients with acute large hemispheric infarction. MATERIAL and METHODS: 33 patients with acute large hemispheric infarction were enrolled, and the systolic peak velocity (Vs), end of diastolic velocity (Vd), mean velocity (Vm), pulsitility index (PI) and percentage rate (PR) of bilateral middle cerebral arteries were determined. RESULTS: (1) The bilateral Vs, Vd and Vm improved significantly from 30 to 120 min after 125-ml mannitol infusion. The PI in the affected hemisphere decreased significantly at 30 min. The improvement of Vs%,Vd%,Vm% and PI% in the affected hemisphere was more evident than in the unaffected hemisphere. (2) Bilateral Vs, Vd and Vm improved significantly from 30 to 90 min after 250-ml mannitol infusion. Bilateral PI decreased significantly from 30 to 60 min. The improvement in Vs%, Vd% and Vm% in the unaffected hemisphere was more obvious than that in the affected hemisphere. CONCLUSION: Single 125-ml mannitol infusion may significantly improve cerebral blood flow and decrease intracranial pressure, and single 250-ml mannitol infusion may markedly decrease the intracranial pressure in the affected hemisphere of patient with acute large cerebral infarction.