The ‘National investigation into the occurrence and effects of estrogenic compounds in the aquatic environment’ (Dutch acronym, LOES), is a base-line study that provides a picture of the occurrence of a number of natural and synthetic estrogens in the aquatic environment as well as a view of the associated estrogenic effects in fish in surface water. The results of the study are described in this report. They provide an objective assessment of the nature and scope of the concentrations measured and the effects on the aquatic environment. A subjective statement on whether a particular effect is ‘bad’ or unacceptable from an environmental perspective will be covered in various policy frameworks at a later stage.