Tresna Dam Reservoir plays a special role in the cascade, acting as a kind of protection of successive water reservoirs on the river Soła. Its trophic condition affects the formation of quality of water in other reservoirs. The aim of the study was the analysis of long-term dynamics of Tresna Dam Reservoir trophic status and the attempt to determine the priority factors of eutrophication. The research is based on water monitoring data carried out during 2000-2015 by Provincial Environmental Protection Inspectorate in Katowice. Assessment of the trophic status is based on ITS indicator (Index of Trophical State), and the results of assessment were verified by Carlson’s indexes TSI (Trophic State Index) and using the Regulation of the Minister of the Environment on the assessment of eutrophication. Convergence of results obtained on the basis of ITS and TSI with the results of evaluations obtained by other authors with different methods allowed to accept the ITS to be reliable indicator of water trophic state, as well as easy and quick method. On the basis of ITS the water of Tresna Dam Reservoir and of feeding the Soła river were assessed as eutrophic. On the basis of the correlation analysis carried out in order to determine the priority factors of eutrophication of the examined water body, it was found that the trophic state is mostly determined by the BZT₅ value, oxygen conditions and the concentration of total phosphorus. The results of the assessement of high trophic status point to the need to protect the reservoir and take steps to avoid eutrophication in both the reservoir and its catchment area, taking into account the priority factors of eutrophication and the role of Tresna in the reservoirs cascade.