This article proposes that the nation undertake a study of a cohort which will be followed from pregnancy to death- a research project that will require more than 100 years. The project would take advantage of the recent completion of the human genome study, the accelerating development of new non-invasive measurement technologies, as well as new information about the complexity and long duration of the causal period for diseases. This complexity involves increasing awareness of long developmental processes which do not fit the typical picture of disease and that do not always have an obvious demarcation of disease onset. Appreciation for the complexity of the web of causation has expanded as the human genome project has unfolded, because it has become increasingly apparent how intimately the action of genetic material depends on contingencies of the individual interacting with the environment; and that the chances of discovering the action of genes, singly or in clusters, will be greatly enhanced by the ability to characterize the environment during distinct developmental periods. Likewise, the ability to understand environmental influences will depend on knowledge of genes. Additionally, there is new evidence for here-to-fore unsuspected comorbidities, the understanding of which would be greatly benefitted by a conception-to-death cohort study with a broad range of health outcomes. In many cases these developmental processes, contingencies, and comorbidities involve long causal periods, approaching that of the entire human lifespan. A conception-to-death cohort study would provide information on disease, human development, environmental risk and protective factors, and public health that will not be achievable by any other research design.