Purpose: The aim of this prospective study was to evaluate the clinical performance of the restorative material Ceram.X in combination with an experimental one-bottle etch-and-rinse adhesive (K-0127). Materials and Methods: A single operator placed two Class I or II restorations in molars of 43 patients. One molar was restored with Ceram.X/K-0127 (Dentsply DeTrey), the other one with Tetric Ceram/Syntac Classic (Ivoclar Vivadent). At baseline, after one, two, and four years the restorations were evaluated by a second dentist using modified Ryge's criteria. After four years, 27 patients were examined. Results: In one patient, both restorations (Class II) had to be removed for root canal treatment due to pulpitis. Another Class II Ceram.X restoration (3.8%; 4.3% [1 of 23] of Class II restorations) showed score C with regard to wear/anatomical form. Thus, the cumulative failure rate was 7.4% in the Ceram.X group (8.3% of Class II restorations [2 of 24]) and 3.7% in the Tetric Ceram group (4.2% of Class II restorations [1 of 24]). Furthermore, three restorations (11.5%) in each group showed score B for anatomical form and marginal integrity. Slight marginal discoloration (score B) was found at five Ceram.X restorations (19.2%) and four Tetric Ceram restorations (15.4%). Two restorations (7.7%) in each group showed slight changes in color stability (score B). No sensitivity, recurrent caries, or changes in surface texture were recorded after four years. No statistically significant differences were found between the two restorative materials (p > 0.05). Conclusion: After four years of clinical service, 92.6% of Ceram.X/K-0127 and 96.3% of Tetric Ceram/Syntac Classic restorations performed clinically well. Keywords: adhesive, ceramic, clinical trial, posterior restorations, resin composite