BACKGROUND This study was performed to evaluate antithrombin III levels in postmenopausal women receiving hormonal replacement treatment. METHODS It is a prospective randomized study concerning 19 postmenopausal patients, aged 40 to 65 years, who received either continuous daily oral equine conjugated estrogen 0.625 mg (group A, N=10) or daily transdermal 17beta-estradiol 50 microg (group B, N=9). Medroxyprogesterone acetate (5 mg/day, 14 days monthly) was given to all patients. Blood samples were obtained before and after 3, 6, 9 and 12 months of treatment. Coagulation tests included Antithrombin III (functional method), prothrombin time, partial activated prothrombin time, thrombin time, factor V, fibrinogen, platelet count and euglobulin lysis time. Friedman analysis of variance and Mann-Whitney test were used for statistical analysis. RESULTS Antithrombin III level was reduced (p<0.05) in group A but not in group B, although it remained within normal range. No changes were detected in the other coagulation tests. CONCLUSIONS These data suggest that oral conjugated estrogen replacement reduces functional ATIII, whereas transdermal estradiol replacement therapy does not modify it.