A dose-response model underlies posttraumatic stress disorder (PTSD) and posits a relationship between event magnitude and clinical outcome. The present study examines whether one index of event magnitudeduration of exposurecontributes to risk of PTSD among female victims of sexual assault. Findings support a small but significant contribution of event duration to clinical status in the immediate aftermath of trauma but not at 3-month follow-up. The opposite pattern is obtained for subjective appraisals of threat. These findings add to a growing literature that suggests that a simple application of the dose-response model to objective event characteristics may be insufficient to explain the risk of PTSD. (Published Abstract)