In this study, patients who had a blood clot in the lung or in a leg vein and completed 6 months of treatment with daily oral vitamin K antagonists (acenocoumarol or warfarin) or once-weekly injections of SR34006 (a new anticoagulant drug) will receive an additional 6 months of once-weekly SR34006 injections or injections of a solution containing no drug (placebo). This trial will evaluate whether patients treated for an additional 6 months with SR34006 have fewer recurrences of blood clots when compared to patients treated with placebo.