B rain arteriovenous malformation (BAVM) rupture is the leading cause of morbidity and mortality in this disease. Predicting outcome after BAVM rupture has mainly relied on demographic, clinical, and imaging markers. 1 S100B serum level elevation has been found to be associated with severity of neurological insult in neurocritically ill patients. 2 The present study aimed to determine whether elevation of S100B is associated with in-hospital mortality after BAVM rupture. Methods Data Availability Study data are available from the corresponding author on reasonable request. Patients Records of patients with BAVM ruptures admitted between January 1, 2003, and December 1, 2018, were retrospectively reviewed. Hemorrhages within 7 days of a partial embolization were excluded. All patients were admitted within 24 hours of hemorrhage. One hundred thirteen ruptures included in this study were previously included in a report of long-term outcome after BAVM rupture. 1 Ethical Statement The ethics committee of our institution approved this study. The need for patients' informed consent was waived. S100B Protein Serum Level Admission S100B serum level and subsequent sampling within the following 48 hours were retrospectively recorded. In most patients, samples were drawn on admission and daily afterwards. Admission samples were unavailable for a minority of patients, and some patients had several samples drawn on the same day (because of acute deterioration for instance). Clinicians were aware of S100B levels in real time. Since December 2, 2010, S100B has been assayed on ModularE170 analyzer (Roche, Mannheim, Germany). Results assessed before that date were measured on a LiaisonXL (Diasorin, Saluggia, Italy); therefore, these values were corrected as ([concentration]-0.01)/2.28, according to the linear regression between the 2 Background and Purpose-S100B protein serum elevation has been associated with poor prognosis in neurologically ill patients. The purpose of this study is to determine whether elevation of S100B is associated with increased in-hospital mortality after brain arteriovenous malformation rupture. Methods-This is a retrospective study of patients admitted for brain arteriovenous malformation rupture. The study population was divided into derivation and validation cohorts. Univariate followed by multivariate logistic regression was used to determine whether elevation of S100B serum levels above 0.5 µg/L during the first 48 hours after admission (S100Bmax48) was associated with in-hospital mortality. Results-Two hundred and three ruptures met inclusion criteria. Twenty-three led to in-hospital mortality (11%). Mean S100Bmax48 was 0.49±0.62 µg/L. In the derivation cohort (n=101 ruptures), multivariate analysis found Glasgow coma scale score ≤8 (odds ratio, 21; 95% CI, 2-216; 0.001) and an S100Bmax48>0.5 µg/L (odds ratio, 19; 95% CI, 2-188; P=0.001) to be associated with in-hospital mortality. When applied to the validation cohort (n=102 ruptures), the same model found only S100Bmax48>0.5 µg/L (odds ratio, 8; 9...