The aim of this study was to establish whether a ‘teach-the-trainer’ course leads to improvements in, firstly, the knowledge and attitude of clinical trainers and their trainees, and, secondly, the role model behaviour of the clinical trainers. A controlled intervention study was performed with GP trainers and GP trainees from four training institutes in the Netherlands. Clinical trainers in the two intervention institutes received two 3-h training sessions on weight management, focusing on knowledge and attitudes towards obesity, and on conveying the correct professional competency as a positive role model for trainees. This was measured using questionnaires on knowledge, attitude, and role model behaviour (the role model apperception tool; RoMAT). GP trainers showed an increase in knowledge and several characteristics could be identified as being related to positive role model behaviour. A small correlation was found between the trainer’s score on the RoMAT and the attitude of the trainee. A teach-the-trainer course in which knowledge, attitudes, and role modelling are integrated proved to be a first step toward improving the knowledge of clinical trainers, but did not result in a measurably better professional outcome for the trainee, maybe due to a more objective level of assessment.