The aim of this study was to evaluate if blood glucose levels change within the first 12 h after stroke onset and to investigate if the degree of change is related to stroke severity, type of stroke, or prognosis. This was a retrospective, descriptive trial based on 445 stroke patients with two blood glucose tests within 12 h of stroke onset and no history of diabetes mellitus. Blood glucose increased in the first 12 h after stroke onset; in mild to moderate stroke from 5.8 to 6.1 mmol/l (P < 0.001) and in severe stroke from 6.2 to 6.7 mmol/l (P < 0.001). In patients who died within 7 days of stroke onset, blood glucose increased from 6.8 to 7.1 mmol/l (P < 0.001). In conclusion, blood glucose increases after the onset of acute stroke and the increase is related to the severity of the stroke.