Cytostatic drugs are compounds used to fight cancer, which may be excreted after administration to patients, and eventually reach wastewater. Given the high incidence of cancer and the properties of drugs, the drug concentrations in water systems should be evaluated. We present the optimization, development and application of a solid phase extraction (SPE) method for the determination of eight cytostatic compounds of different classes in wastewater and seawater samples. We compared four SPE cartridges prior to their determination by Ultra-High Performance Liquid Chromatography tandem Mass Spectrometry. For wastewater samples, the Oasis HLB cartridge gave the best recoveries, which were higher than 65% in most cases, achieving limit of detections (LODs) of 1.68–103.95 ng·L-1 . In seawater samples, the Bond Elut cartridge afforded the best recoveries greater than 70%, with LODs of 0.95–5.14 ng·L-1 .The optimal procedure was applied in four hospital wastewater effluent samples taken during one year, and in different influents and effluents from wastewater treatment plants and seawater from marine outfalls taken in eight campaigns during two years, in Gran Canaria island (Spain). Results showed that etoposide was present in influents of wastewater treatment plants in several months and different wastewater treatment plants and hospital effluents in the range 375.8 – 5141 ng·L-1 , while cyclophosphamide was present in some months in effluents from only one wastewater treatment plant and hospital effluents in the range 55.94 – 1212 ng·L-1 . Vinblastine and vincristine were detected in one sample of hospital at concentrations of 1836 ng·L-1 and 1851 ng·L-1 , respectively.