High blood sugar (hyperglycaemia) affects 40% of acute stroke patients and has a major adverse effect on survival and recovery. Increased production of lactic acid in brain tissue that has a poor blood supply is postulated to be the mechanism by which high blood sugar may worsen brain injury after stroke. Treatment with insulin infusions is proposed as a neuroprotective strategy, and a clinical trial is ongoing to test this hypothesis. However, the biological basis for insulin treatment has not been established, and there is uncertainty about the duration of insulin infusion that may be required to limit damage.