Eleven patients (7 females) with aneurysmal subarachnoid haemorrhage (SAH) and transcranial Doppler (TCD) signs of vasospasm during prophylactic intravenous nimodipine treatment (2 mg/h) were treated with TCD-guided high-dose (4 mg/h) intravenous nimodipine. The patients were followed clinically and with serial TCD investigations. Increasing nimodipine to high-dose treatment led to a reduction of the abnormally elevated mean flow velocities (FV) in all patients. There was also a reversal of clinical signs of delayed ischaemia. In one patient, repeated computer tomographic (CT) investigations revealed a reversal of ischaemic changes. Reduction of nimodipine from 4 to 2 mg/hr resulted in a return to abnormally elevated mean FV as well as a return of clinical signs of cerebral ischaemia. The outcome was favourable in 82% of the patients and there was no mortality or vegetative survival. No patient deteriorated clinically due to vasospasm during treatment with high-dose nimodipine. The individual effect of nimodipine treatment can be monitored by the use of serial TCD investigations. TCD-guided high-dose nimodipine treatment appears to be an effective treatment in SAH patients developing vasospasm despite prophylactic standard dose treatment. The data give support for a direct vascular effect of nimodipine on cerebral vasospasm.