A SPORTIF-III substudy. BACKGROUND: Warfarin has been considered to provide more stable anticoagulant effect than acenocoumarol due to its longer half-life. OBJECTIVE: The aim of this SPORTIF-III substudy was to compare acenocoumarol (A) with warfarin (W) in the same group of 74 patients, with chronic atrial fibrillation who started with W and then changed to A. METHODS: We compared prospectively a 3 months period on W with a 3 months period on A. RESULTS: The mean number of INR measurements per patient was 5.7 +/- 1.2 and 5.4 +/- 1.6 resp (NS). The mean percentage of INR-s in the therapeutic range of 2-3 was 49 +/- 22.6% for W and 56 +/- 26.8% for A (p < 0.05), the percentage of subtherapeutic values were not different, the supratherapeutic values however occurred more frequently on W (28 +/- 20%) than on A (19 +/- 19%), p < 0,001. There was a good correlation between A and W doses (r = 0.65, p < 0.001), the mean W dose was 5.03 +/- 1.99 mg, the mean A dose was 2.5 +/- 1.3 mg, the W/A dose ratio was computed to be 2.18 +/- 0.78. CONCLUSIONS: 1. anticoagulation effect stability was superior for A compared to W; 2. W/A dose ratio was 2.18.