Animal experiments employing controlled degrees of cerebral ischemia have demonstrated that elevated blood-brain glucose concentrations greatly enhance the extent and degree of subsequent brain damage. The question of whether or not a similar relationship applies in man was examined by retrospectively segregating patients admitted with the diagnosis of ischemic stroke into diabetic (n = 35) and nondiabetic (n = 72) groups. A separate nondiabetic population with ischemic stroke was prospectively analyzed by dividing patients into those with an admission blood glucose level above (n = 14) or below (n = 17) 120 mg/dl. The neurologic status at discharge was used to stratify outcome as good, fair, or poor in the retrospective study. The ability or inability to return to work was used to separate good and poor outcomes in the prospective study. Neurologic outcome in diabetic patients with stroke was significantly worse (p less than 0.05) than in nondiabetic patients, and the diabetic patients had a greater (p less than 0.05) number of stroke-related deaths. In the prospective study, neurologic outcome also was worse with high blood sugar levels, only 43 percent of the patients with blood glucose values above 120 mg/dl returned to work, whereas 76 percent of those with lower blood sugar values regained employment (p = 0.061).