Risk of suicide attempt among soldiers in army units with a history of suicide attempts.

In this longitudinal cohort study of administrative records from 9650 medically documented soldiers who attempted suicide between the years 2004 and 2009 and 153 528 control person-months, the risk of suicide attempt among soldiers increased as the number of past-year suicide attempts within their unit increased. This finding was true for combat arms and other occupations and for units of any size but particularly for smaller units (1-40 soldiers).