The aim of this study was to examine the values measured with the Diagnodent®-system for caries detection concerning a dependence on the dryness of the tooth-surface. Measurements were made at 40 points on the occlusal surface of filling-free human extracted teeth stored in physiological saline solution. The teeth, altogether 25 premolars and molars, and the probe of the measurement device were fixed on a mechnical stage, so that their position could be adjusted with a precision of 0.01 mm. All measurements were made at a temperature of 21°C, the relative humidity was 55%. At the beginning of each measurement, the tooth-surface was dried with a gaze-swab. The light-probe A of the Diagnodent®-system was put on the tooth-surface and in temporal intervals of 30 sec the momentary laser-fluorescence value was listed. The duration of all measurement was determined to 22 minutes. In a previous study, it could be shown that dependent on the individual caries risk, a value up to 10 indicates that no dental therapy is necessary. Values of 11 to 30 indicate preventive care and values higher than 30 demand restorative therapy. For this reason the laser-fluorescence values were evaluated in three groups. Within the median values from 0 to 10 (group I), no changes were observed while the drying of the tooth surface took place. In the groups with median values over 10 changes could be observed. Dependent on the period of observation (0.5 to 20 min), the median standard-deviation of the laser-fluorescence-values from 11 to 30 (group II) increased from 0.1 to 1.7. In the group with values over 30 (group III), the median standard-deviation increased from 0.5 to 5.7. Using the laser-fluorescence system for monitoring the progression of a carious lesion, variations of ±2 units can appear when a primary laser-fluorescence value of 11 to 30 was measured. A primary laser-fluorescence value of over 30 can result in variations of ±6 units, which possibly are not caused by the progression of caries but by the unequal dryness of the tooth-surface. Keywords: Laser-Fluoreszenz, Laser, Diagnodent-System, Trockenheit, Austrocknung