Urban firefighters are at risk for posttraumatic stress disorder (PTSD) due in part to their exposure to duty-related trauma. This study compared duty-related trauma exposures and the prevalences of posttraumatic stress in U.S. and Canadian firefighters. Both samples reported relatively numerous and frequent posttrauma symptoms, and the rates of self-reported PTSD prevalence did not differ significantly. However, analysis of departmental records for respondents' previous year on duty revealed significant differences in both frequencies and categories of traumatic incident exposures. Some of the vulnerability and moderating risk factors associated with PTSD caseness differed between the U.S. and Canadian samples. Potential explanations for the observed differences in risk factors for PTSD in these 2 firefighter samples are considered.