The purpose of this study was to compare a high-speed handpiece to an ultrasonic device regarding the time taken for cavity preparation in bovine incisor teeth (n = 12), and to evaluate microleakage after the restorations. Two cylindrical cavities each were prepared on the labial surfaces of 12 teeth. One of them was made with a diamond tip in group 1 (G₁ = 12) coupled with a high-speed handpiece, and the other with a chemical vapor deposition (CVD) tip in group 2 (G₂ = 12) coupled with an ultra-sound device (n = 24). The time taken for each preparation was measured with a stopwatch. The cavities were treated with adhesive (Scotchbond Multipurpose, 3M), restored with composite (Filtek Z250, 3M), finished and polished, and then the prepared teeth were immersed in a 50% silver nitrate solution. The samples were cut in the mesio-distal direction and evaluated for microleakage. Data were analyzed using the Wilcoxon test (P < 0.01). The results revealed that the time taken to prepare cavities was 7.9 times shorter in G₁. There was no microleakage in 100% of the samples indicated by the absence of dye penetration in G₁, and 46% in G₂. It was concluded that the performance of the high-speed handpiece was better than that of the oscillatory system, in terms of both the time taken for preparation and the microleakage. (J. Oral Sci. 49, 207-211, 2007)