Purpose: This longitudinal randomized controlled clinical trial evaluated direct composite restorations for clinical acceptability as posterior restoratives in single- or multi-surface cavities and provides a survey of the 4-year results. Materials and Methods: Three dentists placed 46 Quixfil (Xeno III) and 50 Tetric Ceram (Syntac Classic) composite restorations in stress-bearing Class I and II cavities in first or second molars (43 adult patients). Clinical evaluation was performed at baseline and after 4 years by 2 other dentists using modified USPHS criteria. At the last recall period, 37 Quixfil and 46 Tetric Ceram restorations were assessed. Results: A total of 89.2% of Quixfil and 97.8% of Tetric Ceram posterior composites were assessed to be clinically excellent or acceptable with predominating alfa scores. Up to the 4-year recall, four Quixfil restorations failed because of bulk fracture, partial tooth fracture (2x) and postoperative symptoms. One Tetric Ceram restoration was lost due to problems with tooth integrity. No significant differences between the two composites could be detected at 4 years for any of the evaluated clinical criteria (p > 0.05). The comparison of restoration performance with time within both groups yielded a significant increase in marginal discoloration and decrease in marginal integrity for both materials. After 4 years, small restorations exhibited significantly less marginal discoloration than large restorations. Conclusion: Clinical assessment of stress-bearing Quixfil and Tetric Ceram posterior composite restorations showed good clinical results with predominantly alfa scores for both materials. Keywords: composite, molars, clinical study, longevity, USPHS criteria