We have observed in our study that antithrombin III activity decreases very significatively in eclampsia (p < 0.0001). A level of 90% was defined as a threshold. All the rates which are under or equal to 90% have 78.3% as a positive predictive value and those over 90% have a 98.7% as a negative predictive value for the overcoming of eclampsia. We have concluded that the 90% antithrombin III activity represents the alarm level for over coming eclamptic crises. The determination of the antithrombin III activity must be systematically done in every hypertensive pregnancy with proteinuria.