OBJECTIVE The purpose of this study was to compare the effect of light curing distance on the cytotoxicity of five resin composites cured with three high-power light curing units. METHODS Seven cylindrical discs of each material (Grandio®, Voco; Filtek™ Z250, 3M ESPE; Clearfil™ AP-X, Kuraray Co. Ltd.; Aelite™ LS, Bisco Inc. and Simile®, Pentron) were cured. For curing, soft-up mode of quartz-tungsten-halogen, exponential mode of light emitting diode for 20 s, and ramp-curing mode of plasma arc light curing units for 6 s were used. The curing tip distances were determined as 2 and 9 mm and controlled via the use of metal rings. After ageing the samples for 24 and 72 hours in Dulbecco's Modified Eagle Medium/Ham's F12 (DMEM/F12), cytotoxicity of the extracts to cultured fibroblasts (L 929) was measured by using MTT (tetrazolium salt 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay. The degree of cytotoxicity for each sample was determined according to the reference value represented by the cells in a pure culture medium. Statistical significance was determined using multifactorial analysis of variance. RESULTS The type of resin composite (p <0.05), light curing unit (p <0.05), curing tip distance (p <0.05) and evaluation period factor (p <0.05) had statistically significant cytotoxic effects on L-929 mouse fibroblast cells. However, when the tested materials polymerized at both distances (2 mm and 9 mm) in both evaluation periods (24 h and 72 h), there was no significant difference in the mean CSR% values obtained when the quartz-tungsten-halogen, light emitting diode and plasma arc light curing units were used (p=0.184, F=1.448). CONCLUSIONS The results of this study suggest that the light curing units and resin composites should be harmonized to one another and the curing distance between the tip of the light curing unit and the restoration surface should be as close as possible in order to achieve maximal biocompatibility.