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We measured steroidal estrogens in wastewater in Ottawa and Cornwall (Ontario, Canada) to determine removal efficiency of these steroids during the treatment process, and whether removal varies during a seasonal cycle. Estrone (E1), 17β-estradiol (E2) and 17α-ethinylestradiol (EE2) were found at maximum concentrations in raw sewage (RS), at 104, 66.9 and 5.7ngL⁻¹, respectively. For the Ottawa wastewater treatment plant (WWTP), there was sufficient data to show that E1 concentrations in RS correlated with both ambient air temperature and mean daily flow of the WWTP (R²=0.792, p=0.003 and R²=0.757, p=0.005). E1 removal was correlated with the percent difference in cBOD from RS to FE (final effluent) (R²=0.435, p=0.075). However estrogenic potency, as determined by a sensitive in vitro reporter gene assay, did not decrease during the water treatment process, suggesting that many estrogenic …