Wastes from sewage treatment plants contain synthetic estrogen and similar compounds that are known to affect the reproductive health of exposed fishes in laboratory studies. However, little is known if this pollution could affect populations of fish or other organisms in the wild. ELA researchers added artificial estrogen to a lake for three years and observed the “feminization” of male fish and altered egg production by female fish, leading to the collapse of resident populations. The decline in numbers of small fish, in particular, resulted in negative effects on organisms not directly affected by the added estrogen. These ecosystem changes persisted long after artificial estrogen was detectable in the lake. The study provides guidance to policy makers about the control of synthetic hormones in wastewaters.