This study evaluated fracture resistance in class II slot packable composite restorations with and without the use of a flowable composite liner. A conventional microhybrid composite was used as a control. Thirty-six sound, caries-free human mandibulary molar teeth were used. Separate mesio-occlusal and disto-occlusal class II slot cavity preparations were made in each tooth. The prepared teeth were randomly divided into three groups of 12 teeth. Two packable composites (Surefil, Filtek P60) and one microhybrid composite (Filtek Z250) with their respective manufacturer's bonding agents were used to restore the cavities. One side of each tooth was restored with composite alone, while the other side was restored with the composite lined with that manufacturer's flowable liner. The marginal ridges of the restorations were loaded at an angle of 13.5degrees to the long-axis of the tooth in an Instron Testing Machine until failure. The data were analysed using a one-way ANOVA. There was no significant difference in fracture resistance between composite restorations with and without flowable liners.