This study was designed to evaluate whether the maximum thickness of subarachnoid blood is an independent prognostic marker of mortality after traumatic subarachnoid haemorrhage. Multivariate analysis showed the maximum thickness of subarachnoid blood was an independent predictor of death versus survival 1 month after injury and was inversely associated with Glasgow Coma Scale (GCS) score. Receiver operating characteristic curve analysis showed that maximum thickness of subarachnoid blood > 6.7 mm immediately after non-surgical resuscitation predicted 1-month mortality with 83.9% sensitivity and 67.1% specificity; its predictive value was similar to that of the GCS score. Addition of maximum thickness of subarachnoid blood to the GCS score did not significantly improve predictive performance. Hence, the maximum thickness of subarachnoid blood is a new independent prognostic marker of mortality and might become an additional, valuable tool for risk stratification and decision making in the acute phase of traumatic subarachnoid haemorrhage.