To detect symptomatic hemispheres during the postoperative course of subarachnoid haemorrhage (SAH) using arterial spin labelling (ASL).Eighteen patients with aneurysmal SAH were included; four exhibited symptomatic vasospasm postoperatively. All patients underwent ASL on days 9-10 (single time-point ASL). Nine patients underwent serial measurements of ASL (serial ASL) on days 1-2, 9-10 and 13-21, and seven patients also underwent imaging on days 4-7. CBF in the posterior part of the MCA territory was measured, and the ipsilateral/contralateral ratio of CBF was calculated. Differences between symptomatic hemispheres and others underwent ROC analysis.Single time-point ASL revealed that CBF(day9-10) and CBF(i/c_day9-10) were significantly lower in symptomatic hemispheres than in asymptomatic hemispheres (P < 0.001). Serial ASL was significantly decreased on CBF(day4-7) compared with CBF(day1-2) and on CBF(day9-10) compared with CBF(day4-7), and significantly increased on CBF(day13-21) compared with CBF(day9-10). ROC analysis of single time-point ASL revealed that AUC for CBF(day9-10) was 0.95, significantly higher than CBF(i/c_day9-10) (P < 0.001). ROC analysis of serial ASL showed that AUC for CBF(day9-10) was 0.93 and significantly higher than CBF(day9-10/day1-2) and CBF(i/c_day9-10) (P < 0.001).Single time-point ASL revealed significant CBF reduction in symptomatic hemispheres compared with asymptomatic hemispheres. Serial ASL showed time-dependent CBF changes after SAH.• MR arterial spin labelling (ASL) can non-invasively assess cerebral blood flow (CBF) • ASL revealed significant CBF reduction in symptomatic hemispheres compared with asymptomatic hemispheres • Serial ASL measurements enable observation of time-dependent CBF changes after SAH • ASL is non- invasive and suitable for serial repeated examinations.