Tungsten carbide burs operated at high speeds for trimming and finishing microfilled composite resins are contraindicated as they disrupt the composite resin surface. Diamond burs operated at low speeds do not disrupt the surface on microfilled and small-particle hybrid composite resins. High-speed finishing with carbide burs on small-particle (hybrid) composite resins produces not only a nondisrupted surface but leaves the surface flat and smooth--free from the characteristic striations and grooves left by diamond burs. These results suggest that the rotary instruments used to finish a composite resin must be selected in accordance with the type of composite resins used.