This study tested the proposal that catastrophic appraisals are a risk factor for developing stress reactions after trauma. Trainee firefighters (N = 82) were assessed during training (and before trauma exposure), and 68 firefighters were subsequently reassessed 6 months after commencing firefighter duty (after trauma exposure). Initial assessment included the Clinician Administered PTSD Scale, the Traumatic Events Questionnaire, and the Posttraumatic Cognitions Inventory. The Clinician Administered PTSD Scale was again administered approximately 20 months after initial assessment and after trauma exposure. Posttraumatic stress at follow-up was predicted by pre*trauma catastrophic thinking (24% of variance). These findings accord with cognitive models predicting that a tendency to catastrophize about negative events is a risk factor for developing posttraumatic stress symptoms.