Introduction: Micro-morphological and physicochemical studies on mechanisms of the rapid caries progress after radiotherapy are focused mainly on enamel. Discrimination between defects induced during the irradiation and damage due to subsequent radio-xerostomia is not possible to date. As a consequence, the specific aetiology of radiotherapy induced caries is far from being understood. Objectives: Only few investigations deal with effects of radiotherapy on the dentoenamel junction. This experiment was to study the micro-morphology of the dentoenamel junction and to discriminate between: effects due to the irradiation alone defects induced by radio-xerostomia. The concept of the study was the CLSM visualization of the dentoenamel junction: of sound extracted teeth irradiated extraorally in a Co-60 source of teeth extracted from patients in different phases of radiotherapy. Materials and Methods: Sound, totally impacted and surgically removed teeth, as standards (n = 50) teeth with 33 ± 10 Gy in vivo, no visible defects; (n = 61, 11 patients) teeth with 63 ± 6 Gy in vivo, macroscopically visible defects (n = 147, 24 patients) 500 - 2500 Gy in vitro irradiated teeth, doses 0.1, 1 and 10 Gy/min (n = 30) CLSM is a non-destructive technique for tomographic visualisation of microscopic areas of tooth structures, with the potential of nearly artefact free imaging. Discussion and Conclusions: Radiotherapy induced caries seems to originate from radiation induced defects in the dentoenamel junction. Schlagwörter: radiotherapy, radiation, caries, dentoenamel junction