In this article, we present and discuss an example of how teachers' discussions of mathematical knowledge for teaching (MKT) items elicited their beliefs about the knowledge needed to teach mathematics. One category of MKT is "horizon content knowledge," and this can be described as mathematical knowledge not directly deployed in instruction--or knowledge behind as well as ahead of the pupils in an actual teaching situation. Since teachers' beliefs may influence their teaching and how they approach a professional development course, it is important to study teachers' epistemic beliefs. The transcripts were analyzed in a three-step content analysis approach, and the results indicate that teachers disregard important aspects of knowledge at the mathematical horizon from their teaching knowledge.