This article examines relationships between various dimensions of teachers' professionalism, that is, pedagogical content knowledge and personal characteristics. Using Shulman's notion of pedagogical content knowledge (PCK) we explored the practical knowledge of twenty sex education teachers using in-depth interviews. It appeared that both core elements of PCK, teacher strategy and teacher style (PCK1) and teacher knowledge of student conceptions and learning difficulties (PCK2), are associated with one another in such a way that PCK2 is a prerequisite to PCK1. Furthermore, the PCK of sex education teachers is related to personal characteristics, personality and past experiences with sex education (as a pupil and as a teacher-in-training). (Contains 3 tables and 1 figure.)