A single mutation in the 3'-untranslated region of the prothrombin gene, resulting in a G to A substitution, was recently reported. This finding added to the growing list of genetic disorders thought to be responsible for familial thrombophilia. Although most studies are in general agreement about the elevated risk of venous thrombosis in individuals carrying this mutation, its role in the first event of venous thromboembolism and recurrent events is unclear. Even less clear is the role that this mutation plays in the formation of arterial thrombosis (including coronary artery disease and cerebral ischemia), as studies show contradicting results. Because of this, it is not recommended as part of the routine screening of patients with venous thromboembolism. However, there are certain subgroups of patients who should undergo testing. The discovery of this prothrombin mutation has important clinical implications because it is the second most common cause of genetic thrombophilia, second only to the factor V Leiden. Moreover, its discovery likely will augment the clinician's ability to systematically risk-stratify an individual's likelihood of developing spontaneous thrombosis.