Statement of problem. Several methods have been used to determine the surface characteristics of resin composites in vivo and compare composite wear rates with enamel wear rates. Purpose. This pilot study describes the surface characteristics of resin composites and the wear of resin composites and enamel during 1 year of in vivo service. Material and methods. A single Class II posterior resin composite restoration (Z100) was placed in 10 patients. Restored teeth and unrestored adjacent control teeth were measured for wear 4 times within the first year. A null point contact stylus profiler and fitting software were used to measure epoxy casts. Maximum depth of wear, average depth of wear, and characteristics of the restoration margin were determined. Paired t tests were used to compare the control and restored teeth, and ANOVA was used to assess the progression of wear over time (P<.05). Results. After 1 year, maximum depth of wear over the entire preparation region was on average 204.8 μm (± 129.8), significantly greater than the 36.8 μm (± 10.1) average maximum depth of wear of enamel at occlusal contact areas on control teeth (P=.009). Maximum depth of wear progressed over time (P=.009). Fracture of excess composite, commonly called flash fracture, occurred in 50% of the restored teeth extending over the preparation margin. Conclusion. Composite restorations wore significantly faster than enamel contact areas on control teeth. Also of concern were the marginal flash fractures that could facilitate secondary caries.