The aim of the study was to describe the effects of a caries selective laser system applied on human dentin. Ablation efficiencies of laser pulses emitted from a frequency doubled, gain-switched Alexandrite-laser (wavelength 377 nm) were determined for carious as well as for healthy dentin. Laser pulsed of different pulse duration, 100 ns and 1.6 microsecond(s) , were investigated in comparison. In spite of the different peak power- due to the different pulse durations-the ablation thresholds and ablation efficiencies were found to be about the same for 100 ns as for 1.6 microsecond(s) laser pulses. In healthy dentin two ablation efficiencies (0.03 and 0.12 mm3J-1), according to the two different ablation regimes, could be described. An overall ablation efficiency of carious lesions from 34 different teeth was determined to be about 0.3 mm3J-1. Fluences used for morphological and histological assessment were chosen with respect to the previously described ablation thresholds of healthy dentin. With respect to operating speed, the '99% selectivity mode' for fast caries ablation with fluences within the first ablation regime of healthy dentin (2.6 +/- 0.3 Jcm-2) were used. The effects of different surrounding media-water and air-were studied in comparison. Light- and scanning electron microscopic investigations were performed. If during irradiation the tooth was submerged in water, craters in healthy dentin were small and uncolored. Crater walls demonstrated only a few microns of surface roughness. Ablation debris remains associated to the crater wall. Irradiation in air results in an early carbonization accompanied with melting and cracking of the irradiated dentin. As long as water cooling is provided the histological findings reveal that engaging a frequency-doubled Alexandrite-laser caries selective ablation is possible without too much damage to the surrounding healthy tissues. Nevertheless an active flow of cooling water is desirable in order to remove hot ablation debris out of the crater.