Interobserver variability in the prediction of delayed cerebral ischemia by means of blood on CT was investigated in 159 patients with aneurysmal subarachnoid hemorrhage, admitted within 72 hours after the bleed. The authors found considerable interobserver variability in the assessment of the amount of blood in the individual cisterns. A high sum score was an independent predictor for delayed cerebral ischemia only for rater 1 (rater 1: hazard ratio, 3.26; 95% confidence interval [CI], 1.14 to 7.75; rater 2: hazard ratio, 1.72; 95% CI, 0.72 to 4.09). The authors conclude that interobserver variability limits the predictive power of the amount of blood on CT for the occurrence of cerebral ischemia.