To investigate the effect of conjugated equine estrogens on the coagulability in menopausal women, blood samples were drawn from 69 menopausal females, 35 receiving estrogens and 34 receiving no replacement therapy. Using tests for antithrombin and heparin-antithrombin activities, 14.7% of the control patients were hypercoagulable, while 57.2% of the estrogen-treated women were hypercoagulable. This is a 289% increase in the incidence of hypercoagulability in the group treated with estrogens. The particular relevance of this finding to menopausal women and the importance of the use of the coagulation profile in managing such patients is discussed.