It was recently reported from the Women's Health Initiative that healthy women using combined hormone replacement therapy (HRT) for 5 years have an increased cardiovascular risk. We hypothesize that the increased risk is confined to subgroups of atherosclerotic women. Such women may have higher arterial tissue factor expression and higher thrombin formation, and changes in tissue factor pathway coagulation inhibitor (TFPI) and thrombin activatable fibrinolysis inhibitor (TAFI) may be deleterious. Healthy postmenopausal women (n = 719) were randomized to hormone therapy [n = 357; opposed (n = 290) and unopposed (n = 67)] or no treatment (n = 362). Plasma TFPI and TAFI and the TFPI -287T/C and TAFI -438G/A polymorphisms were measured 5-6 years after randomization. Concentrations of TFPI were significantly lower in the hormone group than in the control group (P < 0.001) and in all genotypes of the TFPI polymorphism. Overall, concentrations of TAFI did not differ between the two groups but were reduced by hormone therapy in homozygotes for the rare TAFI -438 A allele (P < 0.05). The hormone effects on TFPI and TAFI were similar in smokers and non-smokers and in women using unopposed and opposed therapy. The observed decrease in TFPI may contribute to the increased cardiovascular risk associated with HRT.