A major issue in the field within teachers’ professional development research is diagnostic competence of teachers. Past research has shown that teachers' diagnostic competence can be fostered already in the university part of teacher education. In this research, the influence of prospective teachers' professional knowledge on the development of diagnostic competence has been little researched. Therefore, the presented study examines the impact of two specific areas of professional knowledge (content knowledge and pedagogical content knowledge) on the development of diagnostic competence. The results indicate that both knowledge areas have an impact on the development of at least one part of the diagnostic competence.