The weed communities have changed dramatically in the 20th century. Because no weed vegetation relevés for South Moravia early in this century, changes in potential distribution of the most endangered vegetation type, the association Caucalido daucoidis-Conringietum orientalis, are evaluated using records of its diagnostic species. The diagnostic species group of this association was specified by statistical calculation of fidelity. It includes Scandix pecten-veneris, Caucalis platycarpos, Bupleurum rotundifolium, Thymelaea passerina, Nigella arvensis, and Ajuga chamaepitys. The coincidence distribution maps of these diagnostic species were prepared. The resulting maps show that the potential distribution of Caucalido-Conringietum is the South Moravian region of thermophilous flora and possibly the adjacent regions of mesophilous flora. The incidence of this plant association in the area declined remarkably in the second half of the 20th century, but in the 1990s its diagnostic species appeared again in several sites.