In vitro wear of sixteen posterior composite resins was determined by the glass beads abrasion test. The wear was remarkably greater with the thick slurry than with the thin slurry. Microfilled posterior composites were higher wear-resistant than highly loaded and blended posterior composites. There is little difference in wear between submicrofilled resins and microfilled resins with the thick slurry. This suggests that the filler particle size is relatively important to wear resistance of the posterior composite material. There was a close parallel between the results of the glass beads abrasion test and clinical observations. There is no consistency among the wear measurements in the toothbrushing abrasion test.