Increasing recognition that children may be more susceptible than adults to environmental exposures and that they experience potentially life-long consequences of such exposures has led to widespread support for a large new cohort study in the United States. In this article, we propose a framework for a new cohort study of children, with follow-up beginning before birth and continuing to age 21 years. We also describe the administrative structure that has been built to develop the proposal further. The structure includes a partnership between federal and nonfederal scientists and relies on a collaborative, interdisciplinary research effort of unprecedented scale in medical research. We discuss briefly how the proposed cohort could be used to examine, among many other things, the effect of chemical contaminants in breast milk on children's health and development.