Background: Teamwork between clinical teachers is a challenge in postgraduate medical training. Although there are several instruments available for measuring teamwork in health care, none of them are appropriate for teaching teams. The aim of this study is to develop an instrument (TeamQ) for measuring teamwork, to investigate its psychometric properties and to explore how clinical teachers assess their teamwork. Method: To select the items to be included in the TeamQ questionnaire, we conducted a content validation in 2011, using a Delphi procedure in which 40 experts were invited. Next, for pilot testing the preliminary tool, 1446 clinical teachers from 116 teaching teams were requested to complete the TeamQ questionnaire. For data analyses we used statistical strategies: principal component analysis, internal consistency reliability coefficient, and the number of evaluations needed to obtain reliable estimates. Lastly, the median TeamQ scores were calculated for teams to explore the levels of teamwork. Results: In total, 31 experts participated in the Delphi study. In total, 114 teams participated in the TeamQ pilot. The median team response was 7 evaluations per team. The principal component analysis revealed 11 factors; 8 were included. The reliability coefficients of the TeamQ scales ranged from 0.75 to 0.93. The generalizability analysis revealed that 5 to 7 evaluations were needed to obtain internal reliability coefficients of 0.70. In terms of teamwork, the clinical teachers scored residents' empowerment as the highest TeamQ scale and feedback culture as the area that would most benefit from improvement. Conclusions: This study provides initial evidence of the validity of an instrument for measuring teamwork in teaching teams. The high response rates and the low number of evaluations needed for reliably measuring teamwork indicate that TeamQ is feasible for use by teaching teams. Future research could explore the effectiveness of feedback on teamwork in follow up measurements.