We found a new thrombophilic tendency in a family with protein S deficiency. The propositus, a 38-year-old Japanese man, is an offspring of consanguineous marriage and suffered from recurrent episodes of thromboembolism. Hemostatic studies, including platelet counts, platelet aggregation, assays of coagulation factors, and plasminogen activity were all within normal limits. The levels of antithrombin III, alpha 2-macroglobulin, protein C, and protein C inhibitor were also normal. However, functional protein S activity in plasma was markedly decreased (9%) in the propositus. The family study revealed that the reduced levels of functional protein S, less than 5% to 29% of normal (normal range: mean +/- 2 SD of 15 normal adults were 44 to 180%), were found in 11 members of this family over three generations. Six of 11 members had severe protein S deficiency (less than 5%), whereas five had partial deficiency. Four of eight adults with protein S deficiency had recurrent episodes of thrombosis. Immunologic levels of protein S antigen were variable in this family and did not correlate closely with the functional levels. These results suggest that the recurrent thrombotic disease in this family appears to be associated with an inherited deficiency of functional protein S.