BACKGROUND: Much education--especially at the university level--has been criticized for having primarily dealt with explicit knowledge, i.e. those aspects of mental activities, which are verbal and conscious. Furthermore, research in medical diagnostic reasoning has been criticized for having focused on the specialty of intern medicine, while specialties with other skills, i.e. perceptive skills within pathology and radiology, have been ignored. AIMS: To show that the concept of tacit knowledge is important in medical education-at all levels and in medical diagnostic reasoning. METHODS: Describing how tacit knowledge according to Michael Polany, is experienced and expressed in day-to-day life, it is shown that there is a tacit dimension to all knowledge. Reviewing recent literature on medical diagnostic reasoning, it is shown that tacit knowledge is recognized in connection with concepts such as "non-analytical reasoning" and "dual process of reasoning." CONCLUSION: It is important that educators are trained in how explicit and implicit knowledge is attained and that tacit knowledge is included in educational programmes of all medical specialties.