BACKGROUND: Mannitol infusion is widely used in clinical practice to reduce perilesional edema in intracerebral hemorrhage (ICH), though no controlled studies have yet provided evidence of its effects on clinical outcome or on cerebral blood flow impairment following the event. The aim of our study was to evaluate blood flow velocity changes in the middle cerebral arteries (MCA) after a mannitol bolus in patients with ICH. METHODS: Transcranial Doppler bilateral monitoring was performed for 90 min in 20 patients with ICH, during 100 ml mannitol bolus i.v. administration. The MCA mean flow velocities (MFVs) and pulsatility index (PI) were recorded. RESULTS: When the 'healthy' and the 'affected' hemispheres were compared, we observed higher MCA MFV and lower PI on the affected side than on the contralateral side, both at baseline and during the experiment. After the mannitol bolus, we observed a significant MFV increase, starting at the end of the infusion and lasting longer than 60 min in the MCA on the affected side alone. The PI increased after mannitol administration on the healthy side alone. CONCLUSIONS: A single bolus of mannitol modified cerebral hemodynamics in our patients with ICH, increasing flow velocities on the affected MCA. This effect may be a consequence of reduced edema in the perilesional areas. The increased PI on the unaffected side may be indicative of preserved pulsatility in the healthy hemisphere.