We illustrate the strong decrease in the number of butterfly species in Flanders (north Belgium) in the 20th century using data from a national butterfly mapping scheme. Nineteen of the 64 indigenous species went extinct and half of the remaining species are threatened at present. Flanders is shown to be the region with the highest number of extinct butterflies in Europe. More intensive agriculture practices and expansion of house and road building increased the extinction rate more than eightfold in the second half of the 20th century. The number of hot spots decreased considerably and the present-day hot spots are almost exclusively in the northeast of Flanders. Species with low dispersal capacities and species from oligotrophic habitats decreased significantly more than mobile species or species from eutrophic habitats. We discuss these results in a northwest European context and focus on concrete measures to preserve threatened butterfly populations in Flanders.