Objective: The purpose of this in vitro study was to evaluate the effect of light curing unit (QTH or LED) on the marginal seal of composite restorations. Methods: Forty recently extracted human molars were prepared with standardized box-shaped class V cavities of 3.0mm (mesio-distal), 2.0mm (occlusal-gingival) and 2.0mm depth with margins located 1.0mm on enamel and 1.0mm on dentin/cementum, on the buccal or lingual surfaces. The cavities were restored in a bulk increment with Single Bond + Filtek Z250 (3M-ESPE) and randomly assigned to one of the light curing units (n=10): L.E.Demetron I, Demetron-Kerr (DEM), Elipar Freelight, 3M-ESPE (EFL), Ultra Lume LED 2, Ultradent (UL) as the led units and Optilux 501, Demetron-Kerr (OPT), a quartz tungsten halogen curing light that was used as a control. After 24h immersed in tap water the specimens were thermocycled (1000x, 5^o-55^oC, 30s dwell time) and immersed in a 0.5% basic fuchsin solution for 24 hours. The restorations were sectioned longitudinally and evaluated for microleakage using a 0-3 scale. Data were subjected to Kruskal-Wallis test at p<0.05. Results: Median of microleakage scores on enamel margins were: DEM=1; EFL=1; UL=0; OPT=1. On dentin margins median scores were 0 for all groups evaluated. No statistical differences were observed between the groups both for enamel (p=0.3149) and dentin (p=0.2764) margins. Conclusion: Variation of the light curing unit didn't resulted in differences regarding the microleakage of class V composite restorations.

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