Background: In-training assessment has become an important part of clinical teachers' responsibilities. One way to ensure that clinical teachers are qualified for this role is setting up a course. A "Teach the teachers" course focusing on in-training assessment was designed for anaesthesiologists in Denmark. Aims: To evaluate short and longer term effects of a course on in-training assessment for clinical teachers in Anaesthesiology. Method: Fifty-one anaesthesiologists attended a 2-day interactive course about in-training assessment. Effects of the course on knowledge were assessed using identical pre- and post- tests. Longer- term effects were measured six months after the course using the same test. Self-reported use of in-training assessment methods was evaluated using supplemental questions in the follow-up test. Results: There were significant increases in knowledge about in-training assessment immediately following the course (effect size, Cohens d = 1, 5). The knowledge was retained six months later. Knowledge about assessment by clinical structured observation and by written assignments showed further increases in the follow-up period. Participants used the various assessment methods in their daily practice during the six-month study period. Conclusion: A focused "Teach the teachers" course during the implementation phase of a new assessment programme increased participants' knowledge about in-training assessment.