Objective: From 2001 to 2006, the Army deployed over 717,000 personnel to Iraq and Afghanistan, with over 15,000 troops wounded. Little is known about the impact of military and demographic factors, particularly deployment, occupation, and pre-existing medical status, on disability retirement. Methods: A nested case-control study of first time, active duty personnel entering from 1997 to 2004. Cases, individuals granted a medical disability retirement from 1997 to 2006, were identified by the Army Physical Disability Agency. Five controls were matched by year of entrance to each case. Results: Several factors were associated with increased risk of disability retirement, including sex, age, Hispanic ethnicity, body mass index, and military occupation; deployment was associated with a lower risk. Conclusions: The reasons for increased risk among some groups are unknown. The decreased risk associated with deployment probably reflects a "healthy warrior effect," whereas the increased risk for combat arms may reflect combat exposures among the deployed and more rigorous training among the nondeployed.