The Leiden mutation, a point mutation in the gene encoding coagulation factor V, is associated with a high frequency of thromboembolic phenomena. It has recently been connected with adverse outcomes of pregnancy. We carried out this study to define its connection with abruptio placentae. Study Design: Twenty-seven women who had abruptio placentae and 29 control subjects matched for age, parity, and ethnic origin were studied. We studied all women for possible hypercoagulation defects. All women demonstrating resistance to activated protein C were studied for the presence of the factor V Leiden mutation. Results: Seventeen of 27 case patients had an activated protein C ratio ≤2.5, compared with 5 of 29 control subjects (odds ratio 8.16, 95% confidence interval 3.6-12.75, P = .00125). Participants with activated protein C ratios ≤2.5 underwent deoxyribonucleic acid analysis. Eight case patients were found to have the factor V Leiden mutation (5 heterozygous and 3 homozygous, 29.6%), compared with 1 heterozygote among the control subjects who were tested (3.4%). Conclusion: Factor V Leiden mutation was found quite frequently in patients with abruptio placentae. (Am J Obstet Gynecol 1998;179:1565-7.)