Acute hyperglycemia is a common condition among patients with diabetes who are admitted to the emergency department (ED) for acute ischemic stroke (AIS). Previous findings regarding the association between hyperglycemia at admission and adverse outcomes among patients with diabetes and AIS have been inconsistent. When investigating this association, it is necessary to consider premorbid blood glucose control. The objective of the current study was to assess whether HbA1c-based adjusted glycemic variables were associated with unfavorable outcomes among patients admitted to the hospital for AIS. We retrospectively analyzed data from 309 patients who were hospitalized for AIS at a single medical center in Taiwan between January 1, 2013, and October 31, 2015. We found that 1) HbA1c-based adjusted glycemic variables, including the glycemic gap and stress hyperglycemia ratio, were associated with both AIS severity and neurological status at discharge; additionally, 2) HbA1c-based adjusted glycemic variables showed superior discriminative power compared with acute hyperglycemia regarding the development of severe AIS. We conclude that both the glycemic gap and stress hyperglycemia ratio might be useful in assessing the disease severity and prognosis of patients presenting with AIS. Further prospective long-term follow-up studies should be performed to validate these findings.