The representation of oak in forest stands in Slovakia is in the long term declining. In the coming conditions of climate change and especially the arise of long drought periods, we can awaite another degradation of situation of oak stands, which will be guided with appearance of leaf-eating and under-bark insects. New factor is the appearance of half parasite plante yellow mistletoe (Loranhus europaeus Jacq.), which spreads intensive in oak stands in lokalities Považský Inovec, Malé Karpaty, Tríbeč, but also in the sounth of Slovakia and causes the wither or even the perish of tree. The host are mainly the oaks. The yellow mistletoe with the help of sucking roots (haustorias) takes from the body of the host water and mineral substances. Information about yellow mistletoe in the oak forests in the east Slovakia are until now sporadic. The aim of this work is to find out the presence of yellow mistletoe and measure of potential harms in reservation Kokošovská dubina in Slánske vrchy. The bachelor work brings knowledges overview about the influence of yellow mistletoe (Loranthus europaeus Jacq.) and other noxious agents for health stand of oaks and specially pursues the status of oak forests in National Nature Reservation (NNR) Kokošovská dubina.The research work was executed on pernament observation area (POA) 135, founded in 1971. The area has 0,81 hectare and representates typicaly plante community and medial stands conditions in NNR. On the ground of measuring selected growth quantities, the statement of origin and amounth of yellow mistletoe in treetops we have evaluated the health of oaks, which was expressed with degree of parasite attack and with number of yellow mistletoe, which falls to one oak. The measure of yellow mistletoe attack on POA features 33 % with average number 0,81 piece for one oak. The general attack in NNR was estinated from aerial shoots made with the drone. The results shows that it comes 0,57 piece of yellow mistletoe for one tree and the fraction of atacked oaks is 52 %. Mostly attacked were over growen and spread growen oaks and oaks which grow at the edge of stands or in place with clearanced canopy. The harm of trees increased with growing height and thickness. Harmed subjects had in average about 0,4 m lower sitting crown.