Abstract
Subjects with congenital deficiency of protein S, a natural anticoagulant which serves as a co-factor for the antithrombotic activity of activated protein C, are at risk of thrombosis. We describe a family in which deep vein thrombosis -occurring during pregnancy in three members with abnormally low protein S activity - was the only manifestation of congenital heterozygous protein S deficiency. Early diagnosis and active treatment of protein S deficiency should be pursued in any woman with a family history of thrombosis, who is planning pregnancy.