 Background: Metformin treatment has beneficial effects on both glucose and lipid metabolism. Whereas there is general agreement that the blood glucose lowering effect of metformin results from inhibition of hepatic gluconeogenesis, it is less clear exactly how the drug lowers blood triglyceride concentration. There are indications that it enhances hepatic free fatty acid FFA oxidation thus diminishing substrate for reesterification and resecretion as very-low-density-lipoprotein VLDL triglycerides TG. However, the liver is not easily accessible for sampling in humans and data on the clinical effects of metformin in the liver are therefore lacking. This may change due to the increasing use of the positron emission tomography PET technique. Using PET isotopes 11C or 18F coupled to either palmitate or a fatty acid analogue, it is possible to non-invasively measure hepatic fatty acid handling. 