The adverse effect of hyperglycaemia is hypothesised to be consequent to increased provision of substrate to hypoperfused tissue that is metabolising anaerobically, with resultant tissue accumulation of neurotoxic lactic acid. In animal models of stroke, hyperglycaemia causes increased tissue lactic acidosis and increased recruitment of ischaemic tissue in the peri-infarct region into the final infarct. Infarct volumes are higher in hyperglycaemic animals, and conversely, reducing blood glucose reduces infarct volume. Although clinical observational studies suggest protocols that incorporate blood glucose monitoring and control to be beneficial, and trials are ongoing to define the impact of routine treatment to maintain euglycaemia, the basic pathophysiology of stroke in relation to blood glucose has not been well defined in man. Preliminary studies confirm a relationship between blood glucose and lactate concentration in hypoperfused brain tissue but it is unknown whether brain lactate is reduced by control of blood glucose, and whether doing so will impact on stroke evolution. It has also been found that infarct volume increases more in hyperglycaemic patients treated with recombinant tissue plasminogen activator (rtPA).