Introduction: Trauma team activation is a kind of triage the staff of an Emergency Department (ED) exerts to determine whether a trauma team needs to be activated for severely injured patients and in what composition the team needs to be deployed. It is a difficult decision-making process which is often performed by ED nurses and led by guidelines and protocols. There are several factors influencing the decision-making process, such as patient factors, contextual factors and individual factors of the decision maker. Objective: The objective of this study is to understand the trauma team activation decision-making process at a Dutch ED. Secondly, we want to obtain insight in the importance of several factors of influence on the decision-making process for trauma team activation according to ED nurses. Methods: This study has a cross-sectional fractional factorial design. Six patient factors (attributes), of which four with four levels and two with three levels, were identified by literature review and discussed with trauma experts. SPSS® Orthoplan was used to generate a fraction (n=25) of all possible alternative scenarios (n=2304), that consisted of combinations of the attribute levels. Two combinations appeared to be impossible in practice and were removed. Three duplicate scenarios were added to evaluate consistency, but were left out for analysis of importance of the levels. Scenarios were presented to 44 ED nurses at a level I trauma centre using a questionnaire with a total of 26 clinical vignettes. ED nurses were asked to rank the attributes according to their perceived importance. The importance of the attributes and levels was calculated with (normalized) mean rank scores to assess the relative impact of the attributes and levels on the decision-making. A t-test was conducted to calculate the significance of the difference in attribute mean rank scores for two groups with <12.5 and ≥12.5 years of work experience. Consistency of the rank scores among ED nurses was calculated with the intraclass correlation coefficient (ICC). Results: 27 ED nurses completed the questionnaire (response rate 61%). The number of team activations per respondent varied from one to five for normal ED team, from zero to nine for basic trauma team and from nine to 20 for the full trauma team. According to the mean rank score Airway-Breathing (2.68) scored the highest attribute importance, followed by Mechanism of injury (3.19) and Circulation (3.37). After normalizing the level mean rank scores, the level Airway-Breathing unstable was the most important level for trauma team activation based on the relative rank sum weight (0.115), followed by Mechanism of injury Fall of height >5m (0.171) and Airway-Breathing Intubation (0.172). There was no difference in attribute mean rank scores between two groups of ED nurses, stratified for years of work experience. The ICC for the different levels occurring in the three duplicate vignettes varied from 0.432 to 0.795, from 0.712 to 0.802 and from 0.071 to 0.639. Discussion: We observed variation in decisions for trauma team activation and in consistency of the rank scores among the ED nurses under study. This implies that a decision support system could improve uniformity in the trauma team activation decision-making process. With this study we obtained insight in the influence of several patient factors on the decision-making process for trauma team activation. The perceived importance of these attribute levels should be taken into account when a decision-support system is developed. The individual factor years of work experience seemed to have no influence on the perceived attribute importance in this group of ED nurses. In addition, other possible influencing factors such as contextual factors need to be taken into account. To be able to generalize these results, the study needs to be repeated in other (Dutch) EDs.