Concentrations of estrone (E1), 17b-estradiol (E2) and 17α-ethinylestradiol (EE2) were investigated for the first time in sewage effluents and receiving waters of the "Río de la Plata" estuary and neighboring areas by means of LC–MS/MS. E2 and EE2 were ubiquitous in the evaluated sewage effluent samples showing concentrations ranging between 122–631 and 65–187 ng/L, respectively. In surface waters, these estrogens were only detected in the "Girado" stream (Chascomús) at 369 and 43 ng/L, respectively. No significant relationship was found among the size of the served population and the concentration of the estrogens in the sewage effluent. The detection of these estrogens in receiving waters was dependent on the dilution capacity of the system. The studied estrogens were undetectable at the La Plata City water supply station. Conversely, concentrations found at the "Girado" stream indicate a potential ecotoxicological risk of these estrogens to the local aquatic biota.Facultad de Ciencias ExactasCentro de Investigaciones del Medioambiente