Heterozygous protein C deficiency is associated with an increased risk for thrombosis. This association is restricted to a minority of protein C-deficient families, which have been defined as clinically dominant protein C-deficient. In contrast, in the clinically recessive protein C- deficient families, only the homozygous family members are (severely) affected. One possible explanation for this difference in thrombotic risk between families may be the presence of a second hereditary risk factor. A good candidate for this second risk factor is the recently identified resistance to activated protein C (APC). APC resistance, which is associated with a mutation in the FV gene (FV Leiden), is a common and strong risk factor for thrombosis. We show here that the prevalence of the FV Leiden mutation is high among symptomatic protein C-deficient probands (19%). In 6 clinically dominant protein C- deficient families, the segregation of the FV Leiden mutation and the protein C gene mutation was studied. A thrombotic episode had been experienced by 73% of the family members having both the protein C gene mutation and the FV Leiden mutation. In contrast, respectively, 31% and 13% of the family members having either the protein C gene mutation or the FV Leiden mutation had experienced a thrombotic episode. Moreover, the result of a two locus linkage analysis support the assumption that the FV gene and the protein C gene are the two trait loci responsible for the thrombophilia. These results indicate that carriers of both gene defects have an increased risk for thrombosis compared with related carriers of the single defect.