Some investigators suggest that placental thrombosis and infarction can cause recurrent miscarriage. We have shown that the common missense mutation in the factor V gene, the Leiden mutation, which renders factor Va resistant to cleavage inactivation by activated protein C, predisposes to placental thrombosis and spontaneous miscarriage. Our objective was to determine the frequency of the Leiden mutation in a population with well-characterized idiopathic recurrent miscarriage. DNA was extracted from whole blood of 40 couples with a history of idiopathic recurrent miscarriage and 25 couples with a history of proven fertility (seven or more live births). The polymerase chain reaction was used to amplify exon 10 of the factor V gene followed by allele-specific restriction with Mnl1 for mutation detection. Results were analyzed with a chi 2 contingency table. None of the 40 women with idiopathic recurrent miscarriage carried the mutation and only one of their reproductive partners was heterozygous for the mutation. Similarly, none of the control women carried the mutation, and only one of the 25 control male partners was heterozygous for the mutation. In our referral population, the factor V Leiden mutation which predisposes to thrombosis is not a common cause of recurrent miscarriage.