Importance: The rate of suicide attempts in the US Army increased sharply during the wars in Afghanistan and Iraq. Research on this important health outcome has been hampered by the lack of integration among Army administrative data systems. Objective: To identify risk factors for suicide attempts among active-duty members of the regular Army from January 1, 2004, through December 31, 2009. Design, Setting, and Participants: This longitudinal, retrospective cohort study, as part of the Army Study to Assess Risk and Resilience in Servicemembers (STARRS), used individual-level person-month records from Army and Department of Defense administrative data systems to examine sociodemographic, service-related, and mental health predictors of medically documented suicide attempts among active-duty regular Army soldiers from January 1, 2004, through December 31, 2009. We analyzed data from 9791 suicide attempters and an equal-probability sample of 183826 control person-months using a discrete-time survival framework. Data analysis was performed from February 3 through November 12, 2014. Main Outcomes and Measures: Suicide attempts identified using Department of Defense Suicide Event Report records and diagnostic codes E950 through E958 from the International Classification of Diseases, Ninth Revision, Clinical Modification. Standardized estimates of suicide attempt risk for sociodemographic, service-related, and mental health predictor variables were constructed from Army personnel and medical records. Results: Enlisted soldiers accounted for 98.6% of all suicide attempts (9650 attempters; overall rate, 377.0 [95% CI, 369.7-384.7] per 100000 person-years). In multivariate models, suicide attempts among enlisted soldiers were predicted (data reported as odds ratio [95% CI]) by female sex (2.4 [2.3-2.5]), entering Army service at 25 years or older (1.6 [1.5-1.8]), current age of 29 years or younger (<21 years, 5.6 [5.1-6.2]; 21-24 years, 2.9 [2.6-3.2]; 25-29 years, 1.6 [1.5-1.8]), white race (black, 0.7 [0.6-0.7]; Hispanic, 0.7 [0.7-0.8]; Asian, 0.7 [0.6-0.8]), an educational level of less than high school (2.0 [2.0-2.1]), being in the first 4 years of service (1-2 years, 2.4 [2.2-2.6]; 3-4 years, 1.5 [1.4-1.6]), having never (2.8 [2.6-3.0]) or previously (2.6 [2.4-2.8]) been deployed, and a mental health diagnosis during the previous month (18.2 [17.4-19.1]). Attempts among officers (overall rate, 27.9 per 100000 person-years) were predicted by female sex (2.8 [2.0-4.1]), entering Army service at 25 years or older (2.0 [1.3-3.1]), current age of 40 years or older (0.5 [0.3-0.8]), and a mental health diagnosis during the previous month (90.2 [59.5-136.7]). Discrete-time hazard models indicated risk among enlisted soldiers was highest in the second month of service (102.7 per 100 000 person-months) and declined substantially as length of service increased (mean during the second year of service, 56.0 per 100 000 person-years; after 4 years of service, 29.4 per 100 000 person-months), whereas risk among officers remained stable (overall mean, 6.1 per 100 000 person-months). Conclusions and Relevance: Our results represent, to our knowledge, the most comprehensive accounting to date of suicide attempts in the Army. The findings reveal unique risk profiles for enlisted soldiers and officers and highlight the importance of research and prevention focused on enlisted soldiers in their first Army tour.