Introduction Operational stress describes individual behavior in response to the occupational demands and tempo of a mission. The stress response of military personnel involved in combat and peace-keeping missions has been well-described. The spectrum of effect on medical professionals and support staff providing humanitarian assistance, however, is less well delineated. Research to date concentrates mainly on shore-based humanitarian missions. Problem The goal of the current study was to document the pattern of operational stress, describe factors responsible for it, and the extent to which these factors impact job performance in military and civilian participants of Continuing Promise 2011 (CP11), a ship-based humanitarian medical mission. METHODS This was a retrospective study of Disease Non-Battle Injury (DNBI) data from the medical sick-call clinic and from weekly self-report questionnaires for approximately 900 US military and civilian mission participants aboard the USNS COMFORT (T-AH 20). The incidence rates and job performance impact of reported Operational Stress/Mental Health (OS/MH) issues and predictors (age, rank, occupation, service branch) of OS/MH issues (depression, anxiety) were analyzed over a 22-week deployment period. RESULTS Incidence rates of OS/MH complaints from the sick-call clinic were 3.7% (4.5/1,000 persons) and 12.0% (53/1,000 persons) from the self-report questionnaire. The rate of operational stress increased as the mission progressed and fluctuated during the mission according to ship movement. Approximately 57% of the responders reported no impact on job performance. Younger individuals (enlisted ranks E4-6, officer ranks O1-3), especially Air Force service members, those who had spent only one day off ship, and those who were members of specific directorates, reported the highest rates of operational stress. CONCLUSIONS The overall incidence of OS/MH complaints was low in participants of CP11 but was under-estimated by clinic-based reporting. The OS/MH complaints increased as the mission progressed, were more prevalent in certain groups, and appeared to be related to ship's movement. These findings document the pattern of operational stress in a ship-based medical humanitarian mission and confirm unique ship-based stressors. This information may be used by planners of similar missions to develop mitigation strategies for known stressors and by preventive medicine, behavioral health specialists, and mission leaders to develop sensitive surveillance tools to better detect and manage operational stress while on mission. Scouten WT , Mehalick ML , Yoder E , McCoy A , Brannock T , Riddle MS . The epidemiology of operation stress during Continuing Promise 2011: a humanitarian response and disaster relief mission aboard a US Navy hospital ship. Prehosp Disaster Med. 2017;32(4):393-402.