As a result of the dam failure of January 30, 2000 in Rumania, water and a huge amount of sediment contaminated with cyanide and later heavy metals entered the Tisza river system. In order to determine the chronic consequences of the contamination, periphyton and sediment samples were collected from River Tisza and her tributaries over the next 2 years. After flooding periods, the sediment deposited in the foreshore was also sampled. Applying the ICP-AES analysis method, the amounts of major heavy metals in the periphyton and sediment samples were measured, and the related concentration factors were calculated. Attempts were also made to find a correlation with the toxicity data. Ecotoxicological analyses were performed on the sediment using the following test techniques: Daphnia test, static fish test, alga test (chlorophyll content measurements), seedling test (Sinapis) and Lemna test (increase in mass and measurements on the chlorophyll concentration). Examinations on the chronic effects of heavy metals deposited in the periphyton and sediment can be regarded as an important factor in assessing ecological-conservational disasters and in carrying out biomonitoring activities in the future.