ImportanceÂ Mental health of soldiers is adversely affected by the death and injury of other unit members, but whether risk of suicide attempt is influenced by previous suicide attempts in a soldierâ€™s unit is unknown.ObjectiveÂ To examine whether a soldierâ€™s risk of suicide attempt is influenced by previous suicide attempts in that soldierâ€™s unit.Design, Setting, and ParticipantsÂ Using administrative data from the Army Study to Assess Risk and Resilience in Servicemembers (STARRS), this study identified person-month records for all active-duty, regular US Army, enlisted soldiers who attempted suicide from January 1, 2004, through December 31, 2009 (nâ€‰=â€‰9650), and an equal-probability sample of control person-months (nâ€‰=â€‰153 528). Data analysis was performed from August 8, 2016, to April 10, 2017.Main Outcomes and MeasuresÂ Logistic regression analyses examined the number of past-year suicide attempts in a soldierâ€™s unit as a predictor of subsequent suicide attempt, controlling for sociodemographic features, service-related characteristics, prior mental health diagnosis, and other unit variables, including suicide-, combat-, and unintentional injuryâ€”related unit deaths. The study also examined whether the influence of previous unit suicide attempts varied by military occupational specialty (MOS) and unit size. ResultsÂ Of the final analytic sample of 9512 enlisted soldiers who attempted suicide and 151â€¯526 control person-months, most were male (86.4%), 29 years or younger (68.4%), younger than 21 years when entering the army (62.2%), white (59.8%), high school educated (76.6%), and currently married (54.8%). In adjusted models, soldiers were more likely to attempt suicide if 1 or more suicide attempts occurred in their unit during the past year (odds ratios ORs], 1.4-2.3; Pâ€‰<â€‰.001), with odds increasing as the number of unit attempts increased. The odds of suicide attempt among soldiers in a unit with 5 or more past-year attempts was more than twice that of soldiers in a unit with no previous attempts (OR, 2.3; 95% CI, 2.1-2.6). The association of previous unit suicide attempts with subsequent risk was significant whether soldiers had a combat arms MOS or other MOS (ORs, 1.4-2.3; Pâ€‰<â€‰.001) and regardless of unit size, with the highest risk among those in smaller units (1-40 soldiers) (ORs, 2.1-5.9; Pâ€‰<â€‰.001). The population-attributable risk proportion for 1 or more unit suicide attempts in the past year indicated that, if this risk could be reduced to no unit attempts, 18.2% of attempts would not occur.Conclusions and RelevanceÂ Risk of suicide attempt among soldiers increased as the number of past-year suicide attempts within their unit increased for combat arms and other MOSs and for units of any size but particularly for smaller units. Units with a history of suicide attempts may be important targets for preventive interventions.