Arable weeds are among those groups of plants that are most threatened in Europe due to management intensification and efficient cleaning of crop seeds in modern agriculture. Plant species loss in arable fields had been assessed in many European countries about 30 years ago, and has gained renewed interest during the last few years. A rich historical data set on plots where arable weeds had historically been recorded in Switzerland enabled the study of changes in arable weed species since the 1920s onward. In total, 232 locations with historical plots were revisited. There, we recorded all plant species and their abundances on 100m2 plots. Across all plots the average number of species per plot declined dramatically by more than 60% during the last 90 years. Most species decreased in frequency, but common species stayed more frequent, while rare species − often characteristic weeds of traditionally managed crop fields − decreased in frequency or even disappeared. Plant groups with increasing species numbers and frequency were mostly neophytes, grasses and species with high nutrient demand. The above mentioned decline in species number and frequency of rare and characteristic weed species suggests that more effective conservation measures than hitherto taken are needed to ensure their preservation.