The use of posterior composite resins has grown considerably over the past 5 years. While most research efforts have been devoted to improving wear resistance, little effort had been made to determine possible wear of the cusp of the opposing tooth. A recently developed in vitro wear system was used to evaluate a series of composite resins for their ability to abrade opposing enamel c usps. When the resins were tested under the same conditions, a considerable range in enamel wear was observed. Representing opposite ends of the scale, the quartz-containing composite resin caused the enamel to wear away ten times more than did the microfilled composite resin.