INTRODUCTION: Despite its clear association with impaired prognosis, it remains controversial whether hyperglycemia after aneurysmal subarachnoid hemorrhage (SAH) actively contributes to neuronal damage. This study aimed to identify a threshold for blood glucose predicting unfavorable outcome, and to evaluate differences in cerebral metabolism in normo and hyperglycemic SAH patients. METHODS: Prospectively, blood glucose and cerebral metabolism, measured by cerebral microdialysis, were evaluated in 178 patients (WFNS grade I-V; age 51.6 +/- 12.4 years) during days 1-7 after SAH. Patients were classified into groups with mean blood glucose levels <or=/> 6.1 mmol/l (110 mg/dl) and 7.8 mmol/l (140 mg/dl). Glasgow Outcome Score was assessed after 12 months. RESULTS: Higher inpatient blood glucose was associated with impaired prognosis, with a threshold of 7.5 mmol/l (135 mg/dl) distinguishing best between favorable and unfavorable outcome. Inpatient glucose levels >6.1 mmol/l (110 mg/dl) were associated with higher cerebral lactate and lactate/pyruvate ratio (P < 0.05). Cerebral glucose was elevated only at blood levels >7.8 mmol/l (140 mg/dl). Inpatient glucose levels above 7.8 mmol/l (140 mg/dl) were independent predictors of unfavorable outcome and mortality. CONCLUSION: Blood glucose levels >7.8 mmol/l (140 mg/dl), but not levels >6.1 mmol/l (110 mg/dl), independently predicted unfavorable outcome. While blood glucose levels >6.1 mmol/l (110 mg/dl) were already associated with slight metabolic derangements, cerebral glucose increased only at blood levels >7.8 mmol/l (140 mg/dl). Considering the risks associated with tight glycemic control, a moderate regimen accepting blood glucose levels up to 7.8 mmol/l (140 mg/dl) might be more reasonable after SAH.