A comparative flora of ten large (≥ 400 ha) urban parks located in or bordering on Boston, MA; New York, NY; Philadelphia, PA; Baltimore, MD; and Washington, DC was created. Patterns of species presence or absence were analyzed to determine whether a common urban park flora exists; to determine interpark similarities; and the relationship between species diversity and human population of the counties in which the parks are located. The combined vascular flora for the ten parks contains 147 families, 599 genera and 1391 species, 490 of which are non-native. Fewer than 1% of the total number of species were present in all ten parks and less than 2.5% were present in nine or ten parks, indicating that a common urban park flora does not exist. Floristic similarity was related to the geographic proximity of the parks for both native and non-native species. However, the two parks in Bronx County, New York City had greater similarity to parks in Baltimore, MD and Washington, DC than the three parks in Kings and Queens Counties, New York City suggesting that species introductions and removals mask similarity related to geographic proximity. Linear regression analysis revealed a significant direct relationship between human population and non-native species diversity that may be a result of greater access to funds for plantings in more densely populated areas. Also, a significant inverse linear relationship was found between human population and native species diversity, which is likely caused by the higher intensity of trampling and vandalism causing a greater loss of native species.