Many different rotary instruments are available for shaping composite restorations. Whether use of these instruments causes undesirable iatrogenic abrasion of either the tooth surface or the composite restorative material is unknown. Assuming that damage occurs, which technique is least damaging is unknown.This in vitro study quantified the loss of surface enamel and dentin surrounding Class V preparations during composite shaping and finishing procedures. The susceptibility of 2 types of composites to tooth abrasion was also examined.Standardized Class V cavities were prepared at the amelodentinal junction of 36 human molar teeth. The teeth were randomly assigned to 6 groups of 6 teeth each. They were restored with either a low- or high-viscosity composite (Revolution or Prodigy Condensable, respectively) and finished with aluminum oxide disks, tungsten carbide burs, or ultrafine finishing diamond burs. The preparations were profiled before and after restoration. After each finishing procedure, morphological measurements of surface changes in the dentin and enamel were made and reported as volume (in cubic millimeters); maximum depth, mean maximum depth, and mean depth (in micrometers); and surface area (in square millimeters). The results were subjected to a 2-way analysis of variance for restorative material and finishing technique (P<.05).Aluminum oxide disks removed significantly less enamel than tungsten carbide burs or ultrafine finishing diamond burs, as measured by volume, maximum depth, mean maximum depth, mean depth, and surface area (P<.05). Conversely, aluminum oxide disks removed significantly greater dentin than either tungsten carbide burs or ultrafine finishing burs as measured by loss of volume, mean depth, and surface area (P<.05). There was no significant difference in the loss of surrounding tooth substance based on resin type (low or high viscosity).Within the limitations of this study, the 3 finishing systems tested resulted in varying degrees of iatrogenic abrasion of enamel and dentin. The composite material used had no significant effect on abrasion of the surrounding enamel or dentin.