PIP: Many women are now advised not to take birth control pills from 4 to 6 weeks before elective surgery out of concern over serious thromboembolic complications. However, stopping the pill may lead to unwanted pregnancies, and drug prophylaxis for deep vein thrombosis carries risk of morbidity. A study in the 1970s of more than 60,000 British women showed a 4 to 6-fold increase in the relative risk of spontaneous venous thrombosis in young women taking the pill. However, the incidence of spontaneous deep vein thrombosis was remarkably low--43 cases in 23,000 women taking the pill (0.19%) compared with 8 cases in 23,000 women not taking it (0.035%). Since 1968, when the 2 studies were commenced, only 5 deaths (3 of current users and 2 of past users) from pulmonary embolism have been reported. Epidemiological studies have relied almost entirely on cases diagnosed clinically. The clinical diagnosis of deep vein thrombosis after surgery in young women taking the pill (12/1244, 0.96%) was about twice that of women not taking the pill (22/4359, 0.5%), but this difference was not statistically significant. The literature showed 3 studies conducted on young women taking the pill in which Iodine 125 fibrinogen scans were used to diagnose deep vein thrombosis after surgery. The incidences of thrombosis in patients taking the pill were 4.6% in patients who underwent gynecological operations for benign disease, nil in 99 patients who underwent various abdominal operations, and 20% in 33 patients who had emergency appendectomies. Present evidence indicates that the risk to young women of becoming pregnant from stopping the pill or of developing side effects from prophylaxis may be greater than the risk of developing postoperative deep vein thrombosis. It is important to define the true incidence of postoperative deep vein thrombosis so that a rational policy can be adopted. Until such time, the routine use of prophylaxis for deep vein thrombosis in women on the pill is probably unecessary.