BACKGROUND Elevated blood glucose is frequently detected early after aneurysmal subarachnoid hemorrhage (aSAH). We aimed to investigate whether hyperglycemia at admission is associated with mortality in patients with aSAH. METHODS In a multicenter observational study of patients with aSAH, we defined normal glycemia, mild hyperglycemia, moderate hyperglycemia, and severe hyperglycemia as blood glucose of 4.00-6.09 mmol/L, 6.10-7.80 mmol/L, 7.81-10.00 mmol/L, and > 10.00 mmol/L, respectively. We performed propensity score matching to obtain the adjusted odds ratios (OR) with 95 % confidence intervals (CI). RESULTS Of 6771 patients with aSAH, 511(7.5 %) had died in hospital, and hyperglycemia at admission was observed in 4804 (70.9 %). Propensity scores matching analyses indicated that compared with normal glycemia, the odds of in-hospital mortality were slightly lower in patients with mild hyperglycemia (OR 0.89, 95 % CI 0.56-1.40), significantly higher in patients with moderate hyperglycemia (OR 1.90, 95 % CI 1.20-3.01), and in patients with severe hyperglycemia (OR 3.45, 95 % CI 2.15-5.53; P trend < 0.001). Long-term survival was worse among patients with hyperglycemia and was proportional to its severity. Similar dose-response associations were evident for poor functional outcomes and major disability. Hyperglycemia was associated with an increased risk of hospital-acquired infections (OR 1.46, 95 % CI 1.29-1.66) and rebleeding (OR 1.58, 95 % CI 1.06-2.35). CONCLUSIONS Among aSAH patients, hyperglycemia at admission was independently associated with increased mortality. Both moderate hyperglycemia and severe hyperglycemia were associated with an increased risk of mortality, but these associations were not seen in mild hyperglycemia (blood glucose 6.10-7.80 mmol/L).