Individuals with more than one defect in the natural anticoagulant system exhibit an increased risk for thrombosis. We report on a family with two cases of combined protein C (PROC) and protein S (PROS) deficiency, five cases of isolated PROC deficiency Type I, and two cases of isolated PROS deficiency Type I. PROC and PROS deficiency were documented by functional and immunologic tests. The sequencing of all exons and splice junctions of the PROC gene led to the identification of a new, unpublished G-->A transition at nt 8490, leading to an exchange of alanine 259 by threonine. The mutation was present in all family members with PROC deficiency. The carriers of the isolated PROC mutation were asymptomatic at ages of 4, 7, 10, 11, and 80 years. The combination of the PROC mutation with a PROS deficiency in two family members triggered venous thromboembolism at age 31 and 6 years, respectively. The PROS deficiency was associated with complete exclusion of one PROS allele. Two family members with isolated PROS deficiency are still asymptomatic at age 21 and 9 years, respectively. Our findings in this family suggest that the heterozygous mutation at codon 259 of the PROC gene represents a mild thrombotic risk factor and only confers a high thrombotic risk in combination with a second defect, such as the complete exclusion of one PROS allele.