The effect of matrix selection, filler composition, filler silanization, operator variations, and test site (dental clinic) on the wear rate of eight composites were evaluated, The wear was measured on replicas using both a microscopic and a laser scanning measuring method. The average wear rate on contact-free surfaces was 9.2 +/- 4.2 mum/month with the microscopic measurement and 8.5 +/- 3.7 mum/month with the laser scanner over the 36-month period. The urethane-based composites performed significantly better than those which were bisGMA-based. Restorations placed at one dental clinic showed significantly lower initial wear. There was also a significant difference between the operators that was most pronounced during the first 6 months. The other variable (filler composition and silane treatment) did not affect the wear rate significantly.