Background S100B has been described as a biologic marker of neuronal damage. The purpose of this study was to assess its prognostic value in patients with subarachnoid aneurysmal hemorrhage. Methods Seventy-four patients (32 men and 42 women; age, 48 +/- 11 yr) admitted within 48 h after subarachnoid hemorrhage onset and treated by surgical clipping or coiling within 2 days after admission were included. World Federation of Neurological Surgeons, Fisher, and Glasgow outcome scores at intensive care unit discharge and at 6 months were evaluated. Blood concentrations of S100B were determined at admission and daily up to day 8. Results The time course of S100B was increased in patients with high World Federation of Neurological Surgeons and Fisher scores. Patients who underwent surgical clipping had an S100B time course longer than that of those who underwent coiling. This difference remained true after stratification for World Federation of Neurological Surgeons and Fisher scores. The threshold of mean daily value of S100B predicting a poor outcome at 6 months was 0.4 microg/l (sensitivity = 0.50 [95% confidence interval (CI), 0.29-0.71], specificity = 0.87[corrected] [95% CI, 0.76-0.95]). In multivariate analysis, high World Federation of Neurological Surgeons score (odds ratio = 9.5 [95% CI, 3.1-29.4]), mean daily S100B value above 0.4 microg/l (odds ratio = 7.3 [95% CI, 2.3-23.6]), and age (odds ratio = 1.08 per year [95% CI, 1.01-1.15]) were independent predictors of a poor 6-month outcome (Glasgow outcome score 1-3). Conclusion Mean daily value of S100B assessed during the first 8 days is a prognostic tool complementary to initial clinical evaluation in subarachnoid hemorrhage patients.