BACKGROUND Teacher training may improve teaching effectiveness, but it might also have paradoxical effects. Research on expertise development suggests that the integration of new strategies may result in a temporary deterioration of performance until higher levels of competence are reached. In this study, the impact of a clinical teacher training on teaching effectiveness was assessed in an intensive course in emergency medicine. As primary study outcome students' practical skills at the end of their course were chosen. METHODS The authors matched 18 clinical teachers according to clinical experience and teaching experience and then randomly assigned them to a two-day-teacher training, or no training. After 14 days, both groups taught within a 12-hour intensive course in emergency medicine for undergraduate students. The course followed a clearly defined curriculum. After the course students were assessed by structured clinical examination (SCE) and MCQ. The teaching quality was rated by students using a questionnaire. RESULTS Data for 96 students with trained teachers, and 97 students with untrained teachers were included. Students taught by untrained teachers performed better in the SCE domains 'alarm call' (p < 0.01) and 'ventilation' (p = 0.01), while the domains 'chest compressions' and 'use of automated defibrillator' did not differ. MCQ scores revealed no statistical difference. Overall, teaching quality was rated significantly better by students of untrained teachers (p = 0.05). CONCLUSIONS At the end of a structured intensive course in emergency medicine, students of trained clinical teachers performed worse in 2 of 4 practical SCE domains compared to students of untrained teachers. In addition, subjective evaluations of teaching quality were worse in the group of trained teachers. Difficulties in integrating new strategies in their teaching styles might be a possible explanation.