Abstract: The Prothrombin mutation G2021 alone is considered a minor risk factor for thromboembolism, but thromboembolic events are more likely in the presence of additional risk factors. We report on a 44-year-old female with an atrial thrombus causing pulmonary embolism and transiting through a patent foramen ovale. The thrombus was extracted by open heart operation. The patient had a family history for thromboembolic events. Further diagnostic after surgery found the patient positive for the Prothrombin mutation G2021, but not for the Factor V Leiden mutation. After surgery, a permanent oral anticoagulation has been started.