Fascioloidosis is parasitic disease originating in North America caused by the big American liver fluke (Fascioloides magna). Natural and definitive hosts of this parasite are North American deer species. This parasite was first recorded on European territory in Italy in 1875. It spread further by deer game migration, so nowadays it can be found in Austria, Czech Republic, Slovakia, Hungary, Croatia and Serbia. The disease was first identified in Croatia in eastern Baranja, in 1999, when hunters and forest workers noticed change in fitness level in red deer (Cervus elaphus, L.) population and found changes on liver after culling and evisceration of these deer. The field of research and determination of the effectiveness of the therapy for fascioloidosis in deer, is the open hunting ground XVI/11 „Spačva“ located in southeastern Slavonia. The treatment of animals for fascioloidosis on the hunting ground began in late April and early May of 2016. The therapy continued and was conducted in late March and April of the following year, as well as in the first half of February of 2018. After the cull, the livers were eviscerated, sent to the Faculty of Veterinary Medicine in Zagreb, were they were examined. The abundance and developmental stages of fluke Fascioloides magna were determined, as well as the degree of pathoanatomical changes in the liver tissue. In the 2016/2017 hunting season 72 livers of red deer were collected, and in the 2017/2018 hunting season 101 livers were collected. The data is shown in this thesis via tables and charts. Statistical data analysis was made using Statistica 13.5.0.17. program. The results show that the therapeutic suppression of fascioloidosis was effective, even though it was performed in springtime. Effective therapy involved about 52% of the deer population, and the best treatment results are evident in the 6-months-3-year age category. Significant reduction of animals in invasion and superinvasion, as well as increase in reinvasion was recorded during this research, along with the fact that intense foci of fascioloidosis persist in the habitat. A large percentage of reinvasions require the use of other non-therapeutic measures to combat fascioloidosis.