Liners beneath class II composite restorations have been suggested to improve marginal integrity. Objective: It was the aim of this investigation to evaluate the effect of liner placement on microleakage in class II resin restorations. Methods: Ten class II cavities were prepared on the mesial and distal surfaces of extracted third molars for each of 4 groups; (2) resin modified glass ionomer liners, (Fuji Paste Pak, Vitremer), a flowable resin (Point 4 Flowable) and a Control (bonding agent only). Preparations were restored with a resin composite and bonding agent per manufacturer’s suggestion. Samples were finished, stored in distilled water for ³ 24 hrs and then thermocycled for 1000 cycles between 5° and 55° C with a 1-min dwell time. Apices were sealed and teeth varnished to within 1-mm of the margins. Samples were placed in 0.5% basic fuschin dye for 24 hrs, rinsed, embedded in resin and sectioned to produce multiple sections. Microleakage was rated (0-4 ordinal scale) at both the occlusal and cervical margins by consensus of 2 evaluators at 40X. Data were analyzed with Kruskal-Wallis ANOVA for main effect and ranked sums analysis for pairwise testing (a =0.05). Results: There was a statistically significant difference among treatment groups and between cervical and occlusal leakage. Cervical microleakage was significantly greater than occlusal leakage in each system. Median scores at occlusal margins were P (0) < V (0), C (1) F (1), at cervical margins were V (1) < F (2) < P (4), C (4) (@ p< 0.05). Conclusion: In conclusion, RMGI liners placed beneath class II composite restorations yielded significantly less cervical microleakage compared to a flowable resin or bonding agent alone. Supported in part by GC America.