Objectives. The aim of the present study was to evaluate the effect of a thin layer of flowable composite on microleakage in Class II direct packable composite resin restorations on young permanent teeth in vitro. Methods. Twenty sound human premolars and molars extracted for orthodontic reasons were selected for this study. The teeth were randomly assigned into two groups of 10 teeth each (groups A and B). Class II cavities were prepared as uniformly as possible in the mesial and distal aspects of each tooth. The gingival margin extended apically approximately 0.5 mm beyond the cemento-enamel junction, in the dentin. Cavities in group A were restored with packable composite and Alert®/Flow-it® flowable composite, while groups B cavities were restored with Pyramid®/Aeliteflo®. The control cavities in groups A1 and B1 were restored with only packable composite. The teeth were immersed in 2% methylene blue solution for 24 h to allow dye penetration into possible existing gaps between the tooth substance and the restorative material. All teeth were subjected to thermocycling. Results. The dye penetration ranged between 6.6 and 7.2 mm. No significant difference was found between the control and the experimental groups. Conclusion. The use of flowable composite resin as intermediate material does not reduce microleakage.