Coagulation changes and increased risk of thromboembolic disease may occur in association with estrogen administration. The puerperium is also a high-risk period for thromboembolism, and estrogen administration at this time may increase this risk. Patients with congenital deficiency of antithrombin III have recurrent venous thromboembolic disease, suggesting that low levels of this factor may be associated with "hypercoagulability" states. We studied 50 postpartum patients who received chlorotrianisene (Tace) or placebo for lactation suppression in a prospective, double-blind, randomized fashion. Antithrombin III values were significantly lower on the third day post partum in the treated group compared to the placebo group (p less than 0.05). In addition, our clinical data from a total of 99 patients support the previous evidence that estrogens delay rather than prevent breast engorgement. Thus, with questionable benefit and a possible increased thromboembolic risk, it would appear prudent to discontinue the practice of estrogen administration for lactation suppression.