The predictive value of serum cortisol level on the prognosis in acute brain infarction of the carotid circulation territory was studied in 101 patients younger than 70 years. The levels of 7 a.m. and 7 p.m. serum cortisol were measured initially and at 1 week. All patients underwent a computed cerebral tomography (CT) within 2 days of the onset of symptoms, and a second CT 3 weeks or 3 month later. Serum cortisol values predicted the stroke outcome. Both the 7 a.m. and the 7 p.m. values in the initial and 1-week samples correlated positively with the severity of hemiparesis on the corresponding days. The 7 p.m. values predicted better than the 7 a.m. values the functional outcome and case fatality during the 3 month follow-up. Initially and at 1 week, the median 7 p.m. serum cortisol values were statistically significantly higher in those with frontally extending infarcts than in those with non-frontal infarcts. Both 7 a.m. fasting blood glucose and glycosylated hemoglobin (HbA1c) measurements were taken within 3 days of the onset in 95 cases. The patients were diagnosed to have prestroke normoglycemia (n = 73) and hyperglycemia (n = 22) on the basis of the HbA1c level. A highly significant (P = 0.0001) correlation was demonstrated between the initial 7 p.m. cortisol and 7 a.m. fasting blood glucose values in those with prestroke normoglycemia, suggesting that hyperglycemia during the acute phase of stroke is a stress response.