Factor V Leiden and prothrombin 20210 G-A mutations are independent risk factors for venous thrombosis. We studied the frequency of these mutations in 35 patients who had thromboembolic events during pregnancy and puerperium, and in 32 women who had a history of uncomplicated pregnancy, delivered either vaginally or by cesarean section, and did not have a past history of thromboembolism. Factor V Leiden mutation was present in 7 patients (20%) in the study group. Of these 7 patients, 1 was homozygote, whereas the remaining 6 were heterozygote for the mutation. Prothrombin 20210 G-A mutation was present in 2 patients (5.7%) in the study group. In the control group none of the 32 patients was positive for the factor V Leiden and prothrombin 20210 G-A mutations. Our findings indicate that the factor V Leiden mutation is an important risk factor for thromboembolic disease during pregnancy or puerperium.