Frequently Asked Questions
Why does this project use a .org (rather than .mil) website?

The Millennium Cohort Study website address ( uses a .org domain name for complex logistic reasons. It is important to recognize that the study is intended to continue to follow participants even after they leave military service. The project is anticipated to remain DoD-led as well as sponsored, yet it must retain an identity that includes both military and non-military components. For this reason, as well as other technical considerations, the Millennium Cohort Study website is hosted on a .org address. The study team recognizes that .mil websites are more clearly recognizable as authorized DoD sites. All participants should be assured, however, that all Millennium Cohort information security standards meet or exceed those required by DoD websites.

Why is the Millennium Cohort Study being conducted, and what is a cohort?

A cohort study follows a distinct group of people over an extended period of time. In epidemiological research, only a cohort study has the strength to describe long-term health outcomes that are of concern to the military. The Department of Defense determined a need to collect data to evaluate the health of service personnel throughout their military careers and after leaving military service. Important health facts, which may seem obvious now, were not always known to be true - they were learned through cohort studies. For example, we know that smoking causes lung cancer and that untreated high blood pressure causes heart disease. But less than 50 years ago, these relationships were not clear.

Why was this study created and by what authority?

The Millennium Cohort Study is a project recommended by the United States Congress in 1999, and sponsored by the Department of Defense. The study is being conducted by the Naval Health Research Center in San Diego, California. Report Control Symbol # DD-HA(AR)2106, Office of Management and Budget Approval # 0720-0029, and Primary Institutional Review Board Protocol # NHRC.2000.0007.

How will my participation in this study make a difference?

The purpose of this study is to assess the health risks of military deployment, military occupations, and general military service. It is important that we receive your information because you have been selected to represent your fellow soldiers, sailors, airmen, and marines. There are a limited number of Service members taking this survey, so every individual response is needed for this to be a successful study. Ultimately, the greatest benefits of this study will not be known for many years. This is why it is essential to stay involved with the study even after you are out of the service.

Will any members of my Chain-of-Command or Promotion Boards ever see the information that I provide on the questionnaire?

This study is for medical research only. Information you provide is not available to your superiors or promotion boards.

Are there risks to participation?

The study team understands the sensitivity of health-related information. Strict confidentiality of all data on participants is maintained. Data will not be shared with military commands, disability evaluators, insurance companies, or anyone not directly involved in data analysis. All data are maintained securely (see "How Safe is My Information", below). Only summary information from the entire 200,000-member cohort, with no identifying information, will be shared.

I received a survey (or postcard or email), but have since separated from the military, can I still take part?

Absolutely. We need your continued participation even if you are no longer serving in the military. Currently, approximately 58% of the Cohort has left military service. The Millennium Cohort Study is designed to research the effects military service may have on service members' health over a period of many years, including after military service is complete.

How often will I have to submit a survey?

You will be asked to complete one survey, through the mail or online, at least every three years until 2068.

What if I move?

If you move or change duty stations, we ask that you contact us via phone or email to let us know your new contact information.

How safe is my information?

Very secure. The information you provide on your questionnaire is protected and stored in conjunction with a Subject ID number and nothing that would link it to your identity. Only Millennium Cohort Study researchers have access to the data. No personal identifying data will ever be released. All information is stored in locked filing cabinets in double locked rooms on United States Government property as well as additional safeguards for Web-related data. Information stored on computers is protected by all measures required by DoD computer security regulations. Additionally, information on individuals cannot be obtained through FOIA requests, and your insurance companies will not have access to any information that you provide us.

What other information do you obtain about study participants?

We will connect your survey data with other databases, medical records, surveys, or biological specimens collected or maintained by the Department of Defense, Department of Veterans Affairs, federal or state agencies, or nongovernmental organizations such as the National Student Clearinghouse.

We may also utilize commercial sources such as LexisNexis, TransUnion, and other government resources to obtain up-to-date contact information.

Can I participate even though I was not invited?

Thank you for your interest in the Millennium Cohort Study. The scientific validity of this study requires study investigators to follow certain protocols. A random sampling process is used to identify a population to whom invitations are sent. Unfortunately, this means that only those who receive invitations are allowed to participate.

What have the results of the study shown?

Please see our publications page and our 20-year report for just a few of the results and how they are being used to inform policymakers.