Some people are born with genetic or physiologic conditions that make them more likely to develop blood clots in their veins (venous thrombosis). Scientists have recently learned that these congenital problems, which include genetic mutations that affect proteins in blood that regulate clotting, are more common and more varied than had been thought. Researchers have also recently discovered that certain disorders are likely to affect younger people, while other conditions can cause clots at any age. They have also learned that some, but not all, coagulation problems may predispose patients to recurrent blood clots. Since much of the knowledge is new, the authors propose that classifying congenital blood-clotting problems into two major groups may help clinicians diagnose and treat their patients who develop venous thrombosis.