This retrospective study examined risk and protective factors for the development of posttraumatic stress disorder (PTSD) in a sample of 132 Canadian police officers. Structured interviews were conducted in order to assess the most distressing work-related traumatic events and determine diagnoses of full or partial PTSD. Participants completed self-administered questionnaires assessing several potential predictors. The results suggested that 7.6% of the participants developed full PTSD, whereas 6.8% had partial PTSD following an incident at work. A multiple logistic regression analysis indicated that the most potent risk factor for the development of full or partial PTSD was peritraumatic dissociation. Social support from colleagues during the event emerged as a significant protective factor. Clinical implications of the findings are discussed.