The aim of this study was to determine a relationship between various tooth cleaning methods and the values measured with the DIAGNOdent-laser-diagnostic-system for caries detection. Forty filling-free crowns of extracted teeth stored in physiological saline solution were examined. The measurements at four points in every tooth were checked after 1.5 hours for reproducibility. After this the teeth were cleaned using four different methods. Immediately after the cleaning as well as after a further 1.5 hours the measurements were repeated. A cleaning according to all methods investigated did not cause changes of the measured values in points without any plaque or stain. If the tooth surface was changed in visible carious areas or if plaque and stain were removed, the measured values decreased. It could be shown that at apparent caries-free measuring points without any plaque or stain the measured values are not dependent on various tooth cleaning methods. Using the laser-diagnostic-system the teeth should be cleaned before use, to have always the same conditions especially when the system is used for a regular examination of apparent caries-free teeth.