Summary. DNA variations in the Factor V gene have played a major role in thrombosis research ever since the discovery of Factor V Leiden. Here, all relatively common DNA variations in the coding regions of the Factor V gene are discussed. Many of them have been associated with venous thrombosis or related diseases. However, most variations have been studied separately, without taking the presence of other variations in the same gene into account. This means that their association with disease should be interpreted with caution, as it may reflect linkage with another variation. An approach in which a haplotype-based analysis of the Factor V gene is combined with in vitro assays of recombinant proteins is advocated. Finally, a possible reason for the relatively polymorphic nature of the Factor V protein is discussed.