Dabigatran is a new direct competitive inhibitor of thrombin and is equally effective and safe as warfarin in the prevention of thromboembolism in patients with nonvalvular atrial fibrillation. We present a case of a 60-year-old man with persistent nonvalvular atrial fibrillation who switched from acenocoumarol to dabigatran 110 mg twice daily. After five months the patient developed a large atrial thrombus, occlusion of the tibial arteries of the right foot, cerebellar infarction and multiple infarctions in kidneys and spleen. Blood test showed a dabigatran concentration of 35 ng/ml six hours after intake, correlating with a low trough concentration of 24-27 ng/mL and significantly increased thromboembolic risk. Other risk factors for thromboembolism were excluded. The present case indicates that in selected patients, there might be an indication for dose adjustments based on serum levels of dabigatran to ensure patient efficacy (thromboembolic events) and safety (bleeding).