Inherited deficiencies of protein S, an inhibitor of the coagulation system, are now recognized as occurring at least twice as frequently as antithrombin III deficiency in patients with venous thrombosis. Protein S is present in plasma in a complexed form, which is inactive, and in a free or functional form. Free protein S combines with activated protein C to inhibit factors V and VIII. This report describes the evaluation of a family with recurrent deep venous thrombosis and superficial thrombophlebitis. Levels of antithrombin III and protein C as well as plasminogen were normal. The levels of total protein S, which includes the value for the free and complexed forms of protein S, were also normal. However, the free protein S levels were greatly reduced in all symptomatic members who were studied. This report illustrates the importance of obtaining measurement of free protein S levels in patients who are suspected of having inherited venous thrombotic disorders.