An abnormal anticoagulant response in vitro to activated protein C (aPC) has been proposed as an aetiological factor in familial thrombophilia. It is postulated that this phenomenon is due to an inherited molecular defect of factor V resulting in poor inactivation by aPC. We conducted a family study when the proband presented in her second pregnancy with superficial phlebitis, a history of deep venous thrombosis and a family history of venous thromboembolic disease. No abnormality of antithrombin activity, protein C activity or deficiency of protein S were demonstrated in the family members tested. The proband had aPC ratios below the laboratory range on three consecutive occasions. In addition, her mother, who had a history of recurrent DVTs and a pulmonary embolus, and also an asymptomatic nulliparous sister, both had aPC resistance ratios below the laboratory range on consecutive samples. Further information about the combined risk of aPC resistance and pregnancy is needed before guidance on the management of affected women can be formulated.