INTRODUCTION Trauma resuscitations require a coordinated response from a diverse group of health care providers. Currently, there are no widely accepted methods of assessing team effectiveness in this setting. Simulation affords a method to assess team effectiveness. The purpose of this study was to use a simulation setting to develop a specialized assessment instrument for team response in trauma resuscitation. METHODS We developed our assessment instrument using clinical simulation. Four teams of 3 postgraduate year-2 surgical trainees in conjunction with scripted confederates were videotaped enacting 6 separate trauma resuscitation scenarios that mirrored clinical conditions encountered at our level 1 trauma center. Ten of the resulting videotaped scenarios represented a spectrum of team behavior (ineffective to effective) and were scored by 8 experienced clinicians using the Mayo High Performance Teamwork Scale. RESULTS Based in part on the Mayo High Performance Teamwork Scale, we created a prototype trauma team assessment tool consisting of 7 attributes that we scored in binary fashion (present/absent). We validated this prototype by assigning a normalized ranking score to each of the 10 scenarios based on the score supplied by each rater. The presence/absence of the 7 attributes varied significantly among scenarios (52.5% to 93.8%; P < .001). Median scores differed significantly comparing the 5 lowest-ranking scenarios with the 5 highest-ranking scenarios (P < .001). CONCLUSION Our prototype instrument may be effective at assessing team effectiveness during trauma resuscitations. This instrument may prove useful for assessing team competency skills, providing timely feedback to teams, and examining the relationship between effective team function and clinically important outcomes. Further, it may be applicable to other high-acuity, time-sensitive clinical situations.