The Horse-Chestnut Leaf Miner (Cameraria ohridella) is the most important pest of Horse-Chestnut (Aesculus hippocastanum). A pest appears in large populations, because of great adaptability on the various climatic conditions and the lack of natural enemies we can expect his appearance in the all areas of the world where a horse chestnut is growing. The number of generations depends a lot in climatic conditions, and in the area of continental Croatia most often develops three to four generations per year. For successful suppression this pest the first generation controlling is very important because by suppression of a first generation the attack is significantly reduced, and those also the detriment of the future generations. The damages are caused by caterpillars that feed and develop inside of the leaf causing the mines between the two epidermal layers. In a strong attack leaves are becoming brown and it comes to the early defoliation. Repeatedly multiannual early defoliation leads to physiological fading and finally drying and dying trees of Chestnut. There are also an enormous indirectly damages like e.g. the loss of horticultural values. Because of this pest attacks are getting stronger, it's very important to react preventively, on time and professionally to influence on preservation and survival of Horse Chestnut in this area. The research has been conducted during 2016 on five locations in Zagreb (Maksimirska cesta, Okretište Borongaj, Trg maršala Tita, Tuškanac, Trg dr. Franje Tuđmana). On each location visually has been inspected per 20 leaves/5 trees, whereby has been recorded the number of mines on the basis of which it was established the intensity of an attack. The results of tracking were statistically processed, and an intensity of attack comparison was performed at different micro locations in the City of Zagreb. The smallest number of mines (13.1 mines / leaf) was recorded on the site Okretište Borongaj on May 10. The largest number (95.1 mines / leaf) was recorded on Tuškanac on August 30, the highest attacak were registered with third generation on pest.