The proper intensity and illumination time of a curing light is of great importance for the complete polymerization of resin composites and long-lasting resin composite restorations. Inadequately cured resin composites can have a cytotoxic effect on pulp tissue by releasing unreacted monomers. This study determined whether there is any difference in cytotoxicity between composite materials illuminated with different curing modes of LED curing units. Thin layers of two composite materials were polymerized using three different modes of the Bluephase C8 LED curing unit: a high intensity mode (HIP-800 mW/cm 2, 20 seconds), a soft-start mode (SOF-650 mW/cm2 first 5 seconds, 800 mW/cm2 next 25 seconds) and a low intensity mode (LOP-650 mW/cm2, 30 seconds). Lymphocyte cultures were treated with both polymerized and unpolymerized composites using one of the modes stated above. Cells were analyzed using the trypan blue exclusion test, the acridine orange/ethidium bromide dying technique and an alkaline comet assay. Significant cytotoxicity was observed for 120 mg of unpolymerized composites and those polymerized with the HIP polymerization mode. A significant level of DNA damage was detected for 120 mg of unpolymerized composites. However, curing via the LOP program exhibited the lowest genotoxicity. Longer curing time with lower intensity results in less cytotoxicity than shorter curing exposure using a higher intensity of light emitted from the curing light source.