Nonnative species are a major component of global change and have been linked to ecosystem disruption, reduction of native species richness, and substantial economic costs. An understanding of the patterns of occurrence of nonnative species and the environmental correlates of their abundance is necessary to formulate an informed response to the threats posed by these species. While considerable work has been conducted in Europe on the overall patterns and changes in nonnative species, few comprehensive studies are available for other areas. Because patterns of nonnative success can be region-specific, the availability of data from multiple geographic areas is important. Here we use two floristic surveys conducted in the 1930s–1950s and 1980s–2000s, respectively, as a basis for describing patterns and changes in the nonnative vascular flora of Worcester County, Massachusetts. Established, nonnative species comprised 21%–36% of all species in the county's 60 towns. Nonnative species richness was most strongly related to the percentage of land in residential use and was also positively correlated with human population density and with the extent of commercial and urban open land and significantly negatively correlated with elevation and the extent of natural land (mostly forest). Frequencies of nonnative species increased dramatically relative to those of native species between the two sampling periods. The change in the proportion of nonnative flora between the two sampling periods in different towns also showed a significant positive relationship with the change in the percentage of land in residential use. While the frequency of most nonnative species has increased, the frequency of a few species has decreased. Increasing and decreasing species differed in several attributes. Decreasing species had earlier dates of first record than increasing species and were more likely to have European, rather than Asian or North American, native ranges. Decreasers included more agricultural weeds and species of herbal or culinary importance, whereas increasers included more ornamental species. These results suggest minimizing the residential footprint is critical to limiting the spread of nonnative plants and that particular attention should be paid to the invasive potential of ornamental plants.