In 34 girls who were treated with large doses of ethinylestradiol because of expected excessive tall stature, antithrombin activity in the blood was followed before, during, and after treatment. All girls showed lower levels of antithrombin during treatment than before or after estrogen administration; the mean concentration during treatment was at or below the lower limit of control values (-2 SD). In three girls, the antithrombin levels were so low that the medication was withdrawn. There was no difference between the mean levels of antithrombin activity in the three groups treated with 0.25, 0.50, or 1.0 mg of ethinylestradiol. Deficiency of antithrombin is known to be associated with an increased risk of thrombosis. Therefore, antithrombin concentrations in blood should be determined before and during treatment with large doses of estrogens.