Old trees with large girth are important habitats in various ecosystems, and function as cultural legacies as well. Our aims were to measure the greatest (considering girth) trees of Hungary, and compile their main data (including their health status and accessibility). We measured altogether 2000 trees in the area of 531 Hungarian settlements, belonging to 29 native and 43 non-native species. 1550 specimens belong to native species, while 450 are adventive. The most frequent among the greatest tree species is beech (Fagus sylvatica; 400 specimens) and the oak genus (Quercus spp.; 427 specimens). Most of them stand in a park or forest (not as a solitary tree), and live in the Northern Hungarian Mountain Range, while there is a low number of veteran trees in the Great Hungarian Plain. In terms of health status, at least three-quarters of the measured trees are in fair condition. From the registration in the nation-wide online database till our measurement (i.e., between 2000 and 2018) 121 great trees died, mostly due to natural causes (dehydration, storm damage). This number proves that any kind of documentation is important. More than half of the trees are easily accessible, while only about 9% is very difficult to reach. There is only a very limited case when a great tree receives legal protection at the local or national level even on its own or the habitat where it stands.