The formation and maintenance of polyploids (via the development of various reproductive barriers) rank among the central questions of studies on polyploid evolution. However, the long time scale of most evolutionary processes makes the study of the dynamics of diploid-polyploid groups difficult. A suitable candidate for a targeted comparative study is Vicia cracca (Fabaceae), which in the late 1960s was subjected to a detailed cytotype screening in Central Europe. Re-sampling the original localities offers a unique opportunity to assess changes in the ploidy structure of the populations, which should reflect the cumulative effect of all the evolutionary forces acting on the plants. Using flow cytometry, the DNA ploidy levels of more than 6,500 individuals of V. cracca collected at 257 localities in Austria, the Czech Republic, Germany and the Slovak Republic were estimated. Three different cytotypes (2x, 3x and 4x) were detected. While tetraploids predominated in the western part of the area investigated (179 populations), the diploids had a more easterly distribution (62 populations). There is a secondary zone of cytotype contact near the boundary between the Czech and Slovak Republics. Sixteen populations (~6%) consisted of a mixture of 2x and 4x cytotypes. Triploids are very rare; only seven individuals were found in two otherwise diploid populations, indicating the existence of breeding barriers between diploids and tetraploids. The distribution of cytotypes is similar to that determined four decades ago using chromosome counts. Nevertheless, there are some discrepancies, namely the current absence of: (i) the diploid cytotype in southern Bohemia and (ii) the altitudinal segregation in the distribution of cytotypes, including two formerly recognized chromosomal races of diploids, perhaps a result of more representative sampling. Identical monoploid genome sizes (1Cx-values) of both the majority ploidy levels support an autopolyploid origin of the tetraploids.