During the 30-year period of observation, 50, 000 women in Olmsted County had gone through pregnancy. Of these women, 76 had deep vein thrombosis and 24 had pulmonary embolism during their pregnancies or in the 3-month postpartum period. The frequency of venous thromboembolism among these women was 4 times greater than among women in the nonpregnant population. In general, risk for venous thromboembolism increased with each successive trimester of pregnancy but was highest in the postpartum period. In fact, the overall incidence of venous thromboembolism was 4 times higher in the postpartum period than it was during pregnancy, and the incidence of pulmonary embolism was 15 times higher. During pregnancy, younger women had a greater risk of venous thromboembolism than older women; in the postpartum period, the risk was greater among older women. Over the course of the 30-year study, the incidence of venous thromboembolism remained constant during pregnancy but decreased by half among postpartum women.