Objective: Prospective studies have not identified a viable pharmacologic strategy for secondary prevention of posttraumatic stress disorder (PTSD). The authors examined whether preventive intervention via early and short-term administration of a selective serotonin reuptake inhibitor (SSRI), within 1 month of exposure to a traumatic event (before diagnosis of PTSD could be made), may reduce the severity of PTSD symptoms according to DSM-IV at 13 months' follow-up. Methods: Over 25,000 screening calls to patients referred to an emergency department for a traumatic event performed between June 2006 and December 2008 yielded 353 participants who were recruited within the month following a traumatic event. Participants were randomly assigned in a double-blind design to escitalopram (n = 176) or placebo (n = 177). The per-protocol analysis comprised 198 participants (escitalopram, n = 102; placebo, n = 96) who received treatment for 12 to 24 weeks and were available for follow-up at week 56. Results: The primary outcome measure, the Clinician Administered PTSD Scale (CAPS), revealed no prevention effect. However, a secondary outcome, the Pittsburgh Sleep Quality Inventory (PSQI), showed better results for the SSRI group than for the placebo group. For a subset of participants who experienced intentional trauma (missile attacks, rape, or physical assault; n = 50), the prevention effect was found on both primary and secondary measures (CAPS, PSQI and measures of depression and global illness severity). Conclusions: Early and short-term administration of escitalopram was not shown to prevent PTSD, although it did improve sleep quality. In a subgroup of participants who experienced intentional trauma, however, this early-treatment approach may be effective as secondary prevention. This large study is the first to investigate the preventive effect of early administration of escitalopram on PTSD. It highlights the relevance of the type of trauma (intentional vs unintentional) to the outcome.