Abstract Establishing reference conditions is essential for performing ecological status assessments of surface waters according to the EU Water Framework Directive. Ecological status is described as the deviation from undisturbed (reference) conditions. This paper presents the process of establishing preliminary reference values for chlorophyll a, total phosphorus, and Secchi disc readings in different types of Polish lakes. This was done by selecting potential reference lakes and performing statistical analysis of the values of the main eutrophication parameters. The preliminary reference values for total phosphorus, chlorophyll, and Secchi disc depth were defined as 0.033 mgP dm^-3, 3.75 μg dm^-3, and 3.6 m for stratified lakes, and 0.048 mgP dm^-3, 5.80 μg dm^-3, and 2.5 m. for polymictic lakes, respectively.