We report a thrombotic family with combined type I antithrombin deficiency and factor V Leiden (factor V-R506Q) in which the proposita, affected by recurrent venous and arterial thrombosis, was also characterized by mild hyperhomocysteinemia (28 $mUmol/l; normal < 18.5 $mUmol/1). Her two thrombotic sisters, with normal antithrombin levels and factor V molecules, showed hyperhomocysteinemia (51 and 30 $mUol/1, respectively). Four other members of the family had the combined antithrombin/factor V Leiden defect and two of them had thrombosis. The common A223V mutation in the methylenetetrahydrofolate reductase gene, responsible for the thermolabile variant of the enzyme, was found to be heterozygous in the proposita; the two sisters were homozygous and heterozygous, respectively. The heterozygous sister also had a high titre of antiphospholipid antibodies (85 units of immunoglobulin G antiphospholipid antibody/ml). Furthermore, low plasma folate levels were found in the three hyper-homocysteinemic subjects of the family. This family with several prothrombotic defects is a clear example of the polyfactorial nature of thrombophilia. Blood Coag Fibrinol 9:173–176 × 1998 Lippincott-Raven Publishers.