Objective: Hemolysis, elevated liver enzymes, low platelets (HELLP) syndrome is characterized by a distinct activation of the coagulation system. A mutation of the gene coding for coagulation Factor V (Factor V Leiden) has been identified as the most frequent risk factor for thrombosis. To identify risk factors for HELLP syndrome, we determined coagulation parameters and the Factor V Leiden mutation in women who previously had developed HELLP syndrome. Methods: Coagulation parameters (activated protein C resistance, antithrombin, protein C,protein S) were determined in 21 women 6 months to 9 years after they had developed HELLP syndrome in the third trimester. In addition, these women were analyzed for the presence of the Factor V Leiden mutation. Results: Of these analyzed women, 33% (seven of 21) had an activated protein C resistance (activated protein C ratio less than 2.0). Another 38% of the women had subnormal activated protein C ratios (2.0-2.3). Only 57% of the women with an activated protein C resistance were identified as heterozygous carriers of the Factor V Leiden mutation (four of seven). Conclusion: Women with HELLP syndrome have a higher incidence of Factor V Leiden mutations. This increased incidence does not, however, account fully for the increased frequency of activated protein C resistance in these patients. (Obstet Gynecol 1998;92:457-60. (C) 1998 by The American College of Obstetricians and Gynecologists.)