PIP: 50 patients on oral contraception were observed for over 1 year to determine the level of antithrombin 3 and, consequently, the risk of thromboembolism. 12% of patients had lower activity of plasmatic antithrombin 3, and 4 had a diminished level of the antigene antithrombin 3. Dosage of estroprogestational and length of administration can often explain the variations of the level of antithrombin 3. However, in the series here described, the perturbation of the antithrombin 3 seemed to be independent of any dosage or of any length of administration.