Seventy-eight women with a history of thromboembolism were studied for cardiolipin antibodies (CLa), lupus anticoagulant activity (LA) and resistance to activated protein C (APC-resistance). Elevated CLa were found in 15 (19%) and LA in 15 women (19%), respectively. Twenty-six patients (33%) were APC-resistant, 17 of them had LA and/or CLa. No correlation was observed between the LA coefficient, CLa level and the extent of APC-resistance. The study shows the difference in the nature of phospholipid antibodies (PLa) and APC-resistance: cause-dependent for APC-resistance and time-dependent for the LA activity. APC-resistance was commoner in thrombosis triggered by endogenous (pregnancy, delivery) than by exogenous (oral contraceptives, surgery) factors (P < 0.05). The incidence of PLa was not connected to the cause of thrombosis. Time-dependent changes in the PLa spectrum manifested by a reduction of isolated LA frequency five years after thrombosis (P < 0.05). The response of APC seemed to show no variation with time passed after thrombosis. The co-existence of PLa and APC-resistance in 22% of thrombophilic women may stress the role of phospholipids in the blood coagulation process.