The objective of this in vitro study was to evaluate the microleakage in extensive class II preparations in extracted human third molar teeth restored with a visible light-cured composite resin (Visio-Molar) used in conjunction with a low viscosity bonding resin (Visio-Bond) and three cement linings (Ca(OH)2, polycarboxylate cement and glass ionomer cement). Half of the restored teeth were subjected to temperature cycles in water between 5 degrees C and 55 degrees C for 24 h and half were left in distilled water at 37 degrees C for 24 h respectively. The restored teeth were immersed in a fluorescein dye solution and after sectioning the restorations, dye penetration at the restoration tooth interface was assessed and quantified using an optical incident fluorescein microscope. The results showed: 1. Microleakage was observed at the cervical wall of all the restorations. 2. No significant difference in marginal microleakage was found when the restored teeth were subjected to thermal cycling treatment (p greater than 0.05). 3. The use of a glass ionomer cement lining significantly (p less than 0.001) reduced the extent of cervical marginal leakage.