Teacher professional development (TPD) courses can support teachers in building up pedagogical content knowledge (PCK) and in improving the quality of teaching. Therefore, having teachers participate in TPD is of special interest for educational policy. Unfortunately, knowledge about the relation between teachers' PCK and teachers' attendance in TPD is unclear. That's why the current study empirically contrasts teachers (N = 67, German middle-school classes) which do and do not take part in TPD courses exemplarily dealing with formative assessment (FA) in mathematics. Hierarchical cluster analysis was used to identify four clusters of teachers (based on prior personal characteristics concerning formative assessment). Describing these clusters, it can be stated that there exists a cluster of teachers with critical values on all personal characteristics. These teachers mainly do not take part in TPD dealing with FA although they have little PCK concerning FA. Implications for further research and TPD are discussed.