When the Defense Departmentsponsored a health study six years ago, one of its goals was to evaluate the impact of future deployments on longterm health. At the time, investigators did not realize the timeliness of the project.

Today, the Millennium Cohort Study has enrolled tens of thousands of participants who have deployed in support of the war on terror, said Navy Cmdr. Margaret Ryan, the study's principal investigator and director of the Defense Department Center for Deployment Health Research in San Diego last month.

The study was designed in the late 1990s "in the wake of the first Gulf War to answer some of the most difficult questions that couldn't really be answered retrospectively after that conflict," Commander Ryan said.

The joint-service study was established to evaluate the health risks of military deployments, occupational exposures and general military service.

About 108,000 service members have signed up to take part since program enrollment began in July 2001.

Participants' health is evaluated over a 21-year period. And Commander Ryan said the group participating in the study eventually will total more than 147,000 people.

"About 40 percent of our cohort has deployed to one of the more recent operations, either in Iraq or Afghanistan or surrounding regions, in support of the global war on terrorism," she said.

Participants report their health status every three years and can fill out either paper or online surveys, Commander ,Ryan said.

The study is providing valuable data that will help military epidemiologists understand possible cause-and-effect relationships between combat-zone deployments and problems such as posttraumatic stress disorder (PTSD), said Dr. Tyler C. Smith, who will replace Commander replace Ryan as the study's principal investigator later this year.

"We have the ability to look at a large group of individuals who were deployed and not deployed," he said. "We can see what factors predict new-onset PTSD, and how PTSD evolves over time."

Evaluating the incidence of PTSD among service members wasn't possible until recently, "simply because we didn't have a cohort in place like this that's large and population-based," Dr. Smith added (related story, page 26).

More information on the study is available at www.millenniumcohort.org.

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