The objective of this work was to study the prevalence of the Factor V Leiden mutation in an obstetrical clinic largely comprised of African-American women. A cross-sectional study was performed evaluating a total of 231 consecutive women of African-American origin. Of these patients, 21 were considered at high risk for thrombosis, but none was found to carry the mutation. One patient (0.4 per cent) of the total was heterozygous for the Factor V Leiden mutation. African-American women do not appear to be at an increased risk of being heterozygous or homozygous for the Leiden mutation. This low prevalence may be confounded by ascertainment bias in a population of pregnant women.