Millennium Cohort Study: A Department of Defense Research Project, The Millennium Cohort Study is an ongoing longitudinal cohort study headquartered at the Naval Health Research Center in San Diego, California and designed to evaluate any long-term health effects of military service, including deployments. It is the largest population-based prospective health project in US military history, currently collecting data on over 200,000 enrolled participants. Investigators that conduct the Millennium Cohort Study include uniformed and non-uniformed scientists from t.