OBJECTIVES: Warfarin-treated patients with poor international normalized ratio (INR) control, measured with time in therapeutic range (TTR) or the standard deviation of transformed INR (SDTINR), have an increased risk for clinical events. To what extent only a short period with an altered INR control may influence outcomes remains unknown. This study assessed the impact of transient periods of improved or worsened INR control on life expectancy and quality-adjusted life years (QALYs) among warfarin-treated patients with atrial fibrillation (AF) using both metrics. METHODS: Warfarin-treated patients with AF, registered in the patient record system Journalia during years 1985-2000, were included. Information on all-cause mortality was collected from the Cause of Death Register. Scenarios where patients were assumed to have a transiently altered INR control during 30 days were modeled statistically using hazard functions, and the impact on remaining life expectancy and QALYs was assessed. RESULTS: When using SDTINR, a 70-year old man within the 2.5th worst INR control percentile was estimated to gain 10.8 days of life or 0.0168 QALYs from a 30-day improvement in INR control to that of an average 70-year old man. Correspondingly, 15.5 days of life or 0.0196 QALYs would be lost if a 70-year old man within the 2.5th best INR control percentile would have an average INR control during 30 days. The magnitudes were smaller when TTR was used to determine INR control. CONCLUSIONS: Even short periods of altered INR control is expected to have impact on life expectancy and QALYs among patients with AF.