Factor V Leiden (FVL, Arg506Gln) mutation may facilitate embryo implantation and increase fertility and fecundity. This was studied in subjects who were of childbearing age in a time with minimal fertility control without modern contraceptive methods.From 1986 to 1999, 1502 inhabitants of Leiden, The Netherlands, reaching the age of 85 years were enrolled in the Leiden 85-Plus Study. Of 1176 subjects the FVL status was analysed, in 365 male and 811 female subjects.The FVL carrier rate was 4.3%. Fertility was not affected by FVL status. In male subjects, fecundity (interval between marriage and birth of first child) was significantly increased in FVL carriers; 67% of male FVL carriers had a child within 371 days of marriage (therefore conceived within 3 months of marriage), compared with 19% of male non-carriers [relative risk (RR), 3.5; 95% confidence interval (CI), 2.1-5.7; P < 0.001]. Within 6 months of marriage, 75% of male FVL carriers had conceived a child compared with 34% male non-carriers (RR, 2.2; 95% CI, 1.5-3.2; P = 0.01). In female subjects, fecundity was not influenced by FVL status.Fecundity is increased in male FVL carriers; in female subjects, no such association was observed.