BACKGROUND: The Nice terror attack of July 14, 2016 resulted in 84 deaths and 434 injured, with many hospital staff exposed to the attack, either as bystanders on site at the time of the attack ('bystander exposure') who may or may not have provided care to attack victims subsequently, or as care providers to victims only ('professional exposure only'). The objective of this study is to describe the impact on mental health among hospital staff by category of exposure with a particular focus on those with 'professional exposure only', and to assess their use of psychological support resources. METHOD: An observational, cross-sectional, multicenter study conducted from 06/20/2017 to 10/31/2017 among all staff of two healthcare institutions in Nice, using a web questionnaire. Collected data included social, demographic and professional characteristics; trauma exposure category ('bystanders to the attack'; 'professional exposure only'; 'unexposed'); indicators of psychological impact (Hospital Anxiety and Depression Scale); PTSD (PCL-5) level; support sought. Responders could enter open comments in each section of the questionnaire, which were processed by inductive analysis. RESULTS: 804 staff members' questionnaires were analysed. Among responding staff, 488 were exposed (61%): 203 were 'bystanders to the attack', 285 had 'professional exposure only'. The staff with 'professional exposure only' reported anxiety (13.2%), depression (4.6%), suicidal thoughts (5.5%); rates of full PTSD was 9.4% and of partial PTSD, 17.7%. Multivariate analysis in the 'professional exposure only' category showed that the following characteristics were associated with full or partial PTSD: female gender (OR = 2.79; 95% CI = 1.19-6.56, p = 0.019); social isolation (OR = 3.80; 95% CI = 1.30-11.16, p = 0.015); having been confronted with an unfamiliar task (OR = 3.04; 95% CI = 1.18-7.85; p = 0.022). Lastly, 70.6% of the staff with 'professional exposure only' with full PTSD did not seek psychological support. CONCLUSION: Despite a significant impact on mental health, few staff with 'professional exposure only' sought psychological support. Robust prevention and follow-up programs must be developed for hospital staff, in order to manage the health hazards they face when exposed to exceptional health-related events such as mass terror attacks. STUDY REGISTRATION: Ethical approval for the trial was obtained from the National Ethics Committee for Human Research (RCBID N° 2017-A00812-51).< Réduire