Objective: This longitudinal randomized controlled clinical trial evaluated direct composite restorations for clinical acceptability of posterior restoratives in single- or multisurface cavities and provided a preliminary survey of the 3-, 6-, and 18-month results. Method and Materials: Three clinicians placed 46 QuiXfil (Xeno III; Dentsply DeTrey) and 50 Tetric Ceram (Syntac Classic; Vivadent) composite restorations in stress-bearing Class 1 and 2 cavities in first or second molars (43 adult patients). Clinical evaluation was performed at baseline and after 3, 6, and 18 months by 2 other clinicians using modified US Public Health Service criteria. At the final recall period, 45 QuiXfil and 49 Tetric Ceram restorations were assessed. Results: A total of 97.8% of QuiXfil and 100% of Tetric Ceram posterior composites were assessed to be clinically excellent or acceptable with predominating Alpha scores. At the 18-month recall, 1 QuiXfil restoration had failed because of bulk fracture. No significant differences between either composite could be detected at 18 months for all evaluated clinical criteria (P > .05). Small QuiXfil restorations exhibited significantly less marginal discoloration (P = .003) and better restoration integrity (P = .008) than large restorations. The comparison of restoration performance with time within both groups yielded a significant increase in marginal discoloration for QuiXfil (P = .011) and significant deterioration for anatomic form at the marginal step for Tetric Ceram (P = .011). However, both changes were only effects of scoring shifts from Alpha to Bravo. Conclusion: Clinical assessment of stress-bearing QuiXfil and Tetric Ceram posterior composite restorations exhibited for both materials good clinical results with predominating Alpha scores. Keywords: clinical study, composite, longevity, molars, USPHS