In this limited clinical study, 30 posterior adhesive fine-particle hybrid composite resin inlays/onlays were placed. After 12 months in situ, 29 of the restorations were reexamined. The evaluation was carried out using clinical parameters and the scanning electron microscope. None of the restorations showed fractures, recurrent caries, or a clinically visible loss of substance. All restored teeth were vital. The radiopacity of the material was clinically sufficient. Only the color match, because of a suboptimal shade range, and the perfect removal of excess luting composite resin proved to be problematic. Furthermore, slight abrasion of the luting composite resin was observed, although the abrasion had no influence on the quality of the marginal adaptation. In the scanning electron microscope excellent marginal quality was observed initially (98.9% continuous margin) as well as after 12 months (94.9% continuous margin). The evaluated composite resin inlay/onlay system has proved to be reliable during 12 months in situ.