Delayed ischaemic deficits due to cerebral vasospasm contribute to the high morbidity and mortality rates associated with subarachnoid haemorrhage. We evaluated the usefulness of measuring anti-phospholipid antibodies (aPLs) for prediction of the occurrence of symptomatic vasospasm and the outcome after subarachnoid haemorrhage. 32 consecutive patients with subarachnoid haemorrhage due to ruptured cerebral aneurysms were studied. They were admitted and operated on within 72 hours after the onset of subarachnoid haemorrhage. aPLs such as lupus anticoagulants, anti-cardiolipin IgG and anti-cardiolipin IgM were measured repeatedly after admission. Furthermore, platelet count, platelet aggregability and plasma platelet factor 4 were also measured. Eleven among the 32 patients (34.4%) showed positive in the examination for aPLs. Although aPLs could not predict symptomatic vasospasm, once symptomatic vasospasm occurred, patients with aPLs frequently demonstrated cerebral infarction and therefore their outcome was worse. aPLs were associated with a severe initial clinical grade and SAH grade on CT scan. Therefore it may explain the association of aPLs with worse outcome. aPLs were detected between 1 and 7 days. Four of 6 patients (67%) with aPLs became negative between 7 and 13 days after subarachnoid haemorrhage. The mechanism of transient aPLs is unclear but it is more likely to occur in the severer grade patients. The reduction in platelet count, the increased platelet aggregability, and the increased plasma platelet factor 4 concentration were also observed in aPLs-positive patients with symptomatic vasospasm.