Common horse-chestnut (Aesculus hippocastanum) is an introduced plant and has been successfully used for planting for a long time in Lithuania. The state of the A. hippocastanum growing in larger cities was assessed in 1992-2009. Moreover, the assessment was carried out throughout the entire country during 2006- 09. The results of the research described the changing situation of A. hippocastanum during the investigative period: the state was sufficiently good for separate periods, but it began to deteriorate in 1995-97, when a A. hippocastanum was infected with fungi diseases (agents – Guignardia aesculi, Erysiphe flexuosa, and Schizophyllum commune). Unfavourable environmental conditions had also made an influence, whereas horse-chestnut leafminer (Cameraria ohridella) was the most harmful pest over all territory of Lithuania since 2005-06. This invasive pest is common in most European countries. Noteworthy of fallen leaves elimination is the most effective mean restraining abundance of C. ohridella and the spread of fungi diseases.