Cyanobacterial dominance in eutrophic lakes causes water quality problems due to the production of toxins harmful to humans and animals, as well as a number of odorous compounds. Cylindrospermopsin (CYN) is a potent cytotoxic cyanobacterial metabolite involved in triggering illness in humans. The occurrence of CYN has been mostly associated with tropical and subtropical cyanobacteria. We analyzed CYN concentration and phytoplankton assemblages of three lakes located in western Poland during the summers of 2006 and 2007. CYN was detected in 46% of our samples using the HPLC and LC-MS/MS methods. CYN concentrations were in the range of 0.16-1.8 μg L-1 and exceeded the drinking water guideline value of 1 μg L-1 in two samples. This is the first report of CYN occurrence in this part of Europe and provides further evidence that this toxin is common not only in subtropical and tropical regions. The lakes were dominated by Planktothrix agardhii but the occurrence of the CYN investigated here might be associated with the invasive species Cylindrospermopsis raciborskii and/or native Aphanizomenon gracile.