The Leiden mutation in the coagulation factor V (F5) gene associated with preeclampsia in Caucasians has not been found in Japanese populations. We examined the association of 20 missense polymorphisms in the F5 gene in 133 pregnant Japanese women with preeclampsia and in 224 unrelated, healthy, pregnant Japanese women. Among nine polymorphisms identified in the subjects, the M385T and R485K polymorphisms were associated with preeclampsia (P = 0.05 and P = 0.02, respectively). Haplotype analysis indicated that the R485K polymorphism is truly associated with preeclampsia, whereas the association of the M385T polymorphism is due to linkage disequilibrium. Taken together with reports that the R485 allele yields poor factor V function in comparison with that of the K485 allele and that the F5 Leiden mutation is associated with preeclampsia in Caucasian populations, the findings of the present study suggest that the F5 gene is associated with preeclampsia in pregnant Japanese women.