Eutrophication is a serious problem in Polish freshwaters. Mass occurrences of toxic cyanobacteria in reservoirs cause problems in the production of safe drinking water and the diversity of produced toxins complicates monitoring of freshwaters. The aim of this study was to estimate the efficiency of water treatment processes in the removal of microcystins (MCs), cyanobacterial hepatotoxins. Elimination of microcystins was studied at two waterworks, which supply drinking water to the city of Lodz from Sulejow Reservoir. The consecutive steps of pre-oxidation, coagulation, sand filtration, ozonation and chlorination used in the water treatment showed effective elimination of microcystins in water from Sulejow Reservoir in 2002 and 2003. The highest total concentration of microcystin (variants MC-RR, MC-YR, MC-LR) amounted to 6.7 microgl(-1) in raw water and was detected on the 13th of August 2002. In 2003 the water utility decided to increase the contribution of ground water in the production of drinking water. This resulted in a decrease of microcystin in water during and after the treatment process. The current management strategy of the waterworks company includes mixing of surface water and ground water, which reduces the hazards caused by toxic cyanobacterial blooms in the reservoir.