Transcranial Doppler (TCD) examination was performed in 109 patients with aneurysmal subarachnoid haemorrhage. Fifty-seven demonstrated flow velocities exceeding 120 cm/s in the middle cerebral artery. Of these, 23 developed delayed ischaemic deficit (DID). Mean flow velocity in this group was 170, SD 12.8 cm/s, in comparison with 155, SD 11.2 cm/s in the 34 patients without late signs of cerebral ischaemia. This difference is significant (p = 0.0269). In the 34 patients without DID, but TCD > 120 cm/s, 17 received anti-ischaemic therapy based on TCD values only, while 17 were given no additional treatment. The mean TCD values and the neurological outcome in the two groups were similar. A rapid increase in flow velocities of 50 cm/s or more during a 24-h period seemed to be a strong predictor of symptomatic vasospasm as seven out of 12 patients developed DID, five with permanent neurological sequelae. The study confirms results from other centres, that a strict correlation between high TCD flow velocities and occurrence of DID does not exist.