In the course of preparation the inner structure of the hard tooth substance may be damaged by frictioninduced vibrations. This aspect was examined in 30 KaVo high-speed angular handpieces of the new type 2304 LD/24 L by frequency analysis. The individual handpieces showed major differences in the amplitude of the 3rd and 4th harmonic. Above approx. 120,000 revs/min they lead to vibrations in the natural frequency range resulting in measurably smoother running. Solutions are discussed to eliminate these vibrations probably originating in the head of the handpiece.