AF is the most common cause of irregular heart rhythm, becoming more common with age. People with AF may not have any symptoms, but sometimes develop blood clots in the upper chambers of their heart, which may break off and lodge in the brain causing stroke. As a result people with AF are at much higher risk of stroke than others of similar age and sex. The chances of a person with AF having a stroke are greatly reduced if they take anticoagulant drugs, which reduce the tendency of blood to clot. It is therefore important that GPs identify people with AF and that they offer anticoagulant drugs to them to reduce their chances of developing strokes. This analysis will look at a large dataset of electronic patient records from over 500 general practices. It will determine whether the number of people with diagnosed AF is consistent with what we would expect and determine what proportion of these patients are indeed being prescribed anticoagulant drugs.