The number of cytostatic drugs used in cancer treatments is wide and increases every year; therefore, tools have been developed to predict their concentration in the environment to prioritize those for monitoring studies. In the present study, the predicted environmental concentrations (PECs) were calculated according to consumption data in Catalonia (NE Spain) for 2014. According to PECs and to the most widely reported compounds, 19 cytostatics were monitored in two sampling campaigns performed along the Besòs River. A total of seven drugs were detected at levels between 0.5 and 656 ng L. PEC and measured environmental concentrations (MECs) were compared in order to validate PECs. The PEC/MEC ratio presented a good agreement between predicted and measured concentrations confirming the PEC estimations. Mycophenolic acid, prioritized as the compound with the highest PEC, was detected at the highest concentrations (8.5-656 ng L) but showed no risk for aquatic organisms (risk quotient <1) considering acute toxicity tests performed in Daphnia magna.