Considerable progress has been made in determining the molecular bases of inherited thrombophilia in the last 25 years. There are several genetic abnormalities that can be detected in the laboratory and are currently recognized to be unequivocally associated with an increased tendency to develop venous thrombosis. Testing for inherited and acquired causes of thrombophilia may make counseling more focused in ostensibly healthy members of thrombophilic families (i.e., those in which a measurable abnormality was identified in one or more asymptomatic persons). In other situations, including women who start taking oral contraceptives, become pregnant, or consider hormone replacement therapy, screening is generally not useful. It is hoped that because we currently have such simple and clinically useful global tests of hypocoagulability as the prothrombin time and activated partial thromboplastin time, it may soon become possible to measure multifactorial thrombophilia without resorting to the multiplicity of tests now necessary.