Abstract Objectives This controlled prospective split-mouth study evaluated the clinical behavior of two different resin composites in extended Class II cavities over a period of four years. Methods Thirty patients received 68 direct resin composite restorations (Grandio bonded with Solobond M: n = 36, Tetric Ceram bonded with Syntac: n = 32) by one dentist in a private practice. All restorations were replacement fillings, 24 cavities (35%) revealed no enamel at the bottom of the proximal box, in 33 cavities (48%) the proximal enamel width was less than 0.5 mm. The restorations were examined according to modified USPHS criteria at baseline, and after six months, one, two, and four years. At each recall, impressions were taken for replica preparation. Replicas of 44 select subjects were assessed for marginal quality under a stereo light microscope (SLM) at 130× and 22 replicas were assessed under a scanning electron microscope (SEM) at 200×. Results Both recall rate and survival rate were 100% after four years of clinical service. No significant difference was found between the restorative materials ( p > 0.05; Mann–Whitney U-test). Hypersensitivities were significantly reduced over time ( p < 0.05; Friedman test). A significant deterioration over time was found for the criteria marginal integrity (66% bravo after four years), tooth integrity (15% bravo), filling integrity (73% bravo) and proximal contact ( p < 0.05; Friedman test). SLM and SEM analysis of restoration margins revealed differences in the amount of perfect margins, in favor of Tetric Ceram ( p < 0.05). Significances Both materials performed satisfactorily over the four-year observation period. Due to the extension of the restorations, wear was clearly visible after four years of clinical service with 50% bravo ratings.