OBJECTIVES: To study warfarin compliance by Australian patients with Atrial Fibrillation (AF). METHODS: Longitudinal assessment of Pharmaceutical Benefit Scheme (PBS) claim records provided by Medicare Australia. We analysed a 10% random sample of all Australian long-term health concession card holders who had taken an anti-arrhythmic medication for AF prior to a new initiation of warfarin (time window October 2006 to September 2007), and for whom warfarin had not been dispensed in the 12 months prior to the initiation. Regimen complexity was assessed using the number of strengths of warfarin taken in a 90-day period. Treatment compliance was assessed using: the proportion failing to fill a second prescription; median persistence time with medication: long term persistence at 30 months. Treatment cessation was defined as 6 months without a warfarin prescription. RESULTS: The PBS Claims database yielded information on 1,108 concessional AF patients newly initiated on warfarin. These AF patients were initiated on 2.4 different strengths of warfarin in the first 3 months and the regimen complexity was 2.0 strengths 12 months later. Australian PBS claims data showed that 13% stopped warfarin after the first script. Median persistence to warfarin was 22 months, while long term persistence (30 months) was around 42%. Decreasing the cessation criteria from 6 to 3 months revealed large treatment gaps in 16% of warfarin patients at 2 years.    CONCLUSIONS: There is a significant problem with complex dosing regimens and poor persistence in AF patients taking warfarin. Warfarin regimens tend to be complex with AF patients averaging 2 or more strengths of warfarin. Not only do the majority of AF patients stop warfarin within 2 years, a further one in six had large treatment gaps. If AF patients fail to take their anticoagulation therapy regularly, then this will substantially increase the risk of adverse outcomes including stroke.