OBJECTIVES Cavity lining with flowable composites has been proposed to reduce interfacial stress due to its strain-absorbing capacity. The purpose of this in vitro study was to evaluate marginal and internal adaptation of large Class I restorations filled in bulk after cavity lining with flowable composites. METHODS Forty standardised large Class I cavities with enamel margins not supported by dentin, were randomly assigned to four groups and either restored in bulk after lining with flowable composites or in two oblique layers of high-viscosity composites. Two adhesives and flowable resins were used: Tetric Ceram, Tetric Flow, Excite (Vivadent, Schaan, Liechtenstein) vs. Prodigy Condensable, Revolution, Optibond Solo (Kerr Co., Orange, CA, USA). Immediately after restorative procedure marginal quality and the internal restoration interface of two bucco-oral sections were quantitatively assessed by SEM using replica technique. RESULTS Marginal gaps were almost not seen in all test groups (<0.001%), whereas marginal enamel fractures frequently occurred (>23%). No differences in marginal adaptation were detected for both application techniques if the same adhesive was used (ANOVA, Tukey, p<0.05). Lining with Revolution resulted in a significantly higher percentage of marginal continuity (MC) and less enamel fractures (EF) compared to Tetric Flow (MC: 73 vs. 45%; EF: 23 vs. 50%). Gaps at the inner interface were rarely observed (<0.05%). CONCLUSIONS Large Class I restorations without dentin supported cavity margins showed a high amount of marginal enamel fractures directly after placement. Lining with Revolution proved to have the highest potential to initially maintain the marginal integrity.