This study evaluated the microleakage of endodontically treated teeth restored with prefabricated dowels and tooth-colored restoratives (Solitaire, Surefil, Filtek P60, Admira, Z-100) as core materials with and without the use of a flowable composite liner. A total of 200 extracted premolars were endodontically treated and prepared for post-core restorations. Prefabricated posts were inserted and cemented with resin cement into the root canals. Each resin system included the corresponding bonding agent for each respective flowable composite. For each group, half of the teeth were restored with composite alone, while the others were restored with the composite lined with flowable liner. Teeth were thermocycled for 1,000 cycles between 5 and 55°C using a 30-second dwell time. The teeth were then coated with nail polish, placed in 5% methylene blue for 24 hours, sectioned, and scored for microleakage. Flowable liners reduced microleakage. Z-100 both with and without flowable liner demonstrated better resistance to leakage. Solitaire, Admira, and Filtek P60 showed more leakage than the other materials.