Mild hyperhomocysteinaemia has been identified as a risk factor for arterial disease and venous thrombosis. In 1991, elevated homocysteine concentrations were also suggested to be associated with neural tube defects (NTD). Since then this relationship has been reported frequently, as well as the relationship between NTD and the 677 C-->T mutation in the methylenetetrahydrofolate reductase (MTHFR) gene. Meanwhile, disturbances in the homocysteine metabolism have also been reported as a risk factor for early pregnancy loss and for other congenital birth defects. However, besides embryonic or foetal consequences, hyperhomocysteinaemia has also been described as a cause of maternal obstetric complications such as pre-eclampsia. This review is concerned with the role of hyperhomocysteinaemia in human reproduction.