The presence of endocrine-derived compounds in the environment occurs due to a myriad of human or industrial activity and can disrupt the endocrine system of animals, including fish. One important group of endocrine disruptors are the estrogens, such as 17-β estradiol (E2, estradiol). Estrogens are gonadal steroid hormones, able to be influential even in small concentrations. Here, we demonstrate that E2 is linked to female’ decisions made by an important coral reef species, the cleaner fish Labroides dimidiatus, during interactions with other reef fishes (known as clients). E2 treatment in natural conditions interfered directly in the cooperative relationships, by increasing cleaners’ willingness to interact with clients, providing greater amounts of physical contact to their fish partners. We discuss the meaning of the observed behavioural disruption produced by E2, which by affecting a key species (cleaners) may produce a cascade impact in the aquatic ecosystem.