Estrogen-like chemicals, so-called xenoestrogens, have become a topic of concern because they are potentially capable of disturbing the hormonal balance of wildlife and humans. Effluents of wastewater treatment plants (WWTPs) are presumably the major source of xenoestrogens in the aquatic environment. In this study, we investigated eight WWTPs with respect to their input, elimination efficiency, and output of estrogenic activity by means of a reporter gene-based bioassay. All WWTPs employed activated sludge treatment with nitrification/denitrification and tertiary treatment (second nitrification and/or filtration). Estradiol equivalents (EEQs) in the influents of the WWTPs were between 5.7 and 65.8 ng/L. The greatest inputs were found in plants treating pure domestic sewage and in samples collected in winter. Process waters either had no estrogenic activity or EEQs in the range of raw sewage, depending on the source of the process water. EEQs of effluents ranged from mostly below quantification limit (0.8 ng/L) to a maximum of 5.4 ng/L in secondary and 1.4 ng/L in tertiary effluents. These findings demonstrate the elimination efficiency of the activated sludge treatment and the further improvement by additional tertiary treatment. However, several concentrated effluents elicited little, but detectable estrogenic responses in the bioassay.