Objective: To determine the effect of delayed start of light polymerization of a dual-cured composite base on the microleakage of Class 2 open-sandwich composite restorations. Method and Materials: Fifty extracted human molars were used to prepare Class 2 mesio-occlusal and disto-occlusal slot preparations. Teeth were randomly divided into 2 groups and restored with a base of dual-cured composite in the proximal box and a top layer of light-cured composite. Group I was restored with a 1-step dual-cured bonding agent; group II was restored with a 2-step dual-cured bonding agent. Five subgroups were created according to the method of polymerization of the dual-cured composite: (A) self-cured, (B) light-cured immediately, (C) light-cured 30 seconds after placement, (D) light-cured 60 seconds after placement, and (E) light-cured 120 seconds after placement. Restorations were stored for 1 week at 37°C and 100% relative humidity, thermocycled (2,000 times, 5°C to 55°C, 15-second dwell), and immersed in a 1% aqueous solution of methylene blue for 24 hours at 37°C. Samples were sectioned mesiodistally, and dye penetration at enamel, dentin, and cementum margins was scored under a stereomicroscope using an ordinal scoring system. Results: Statistical analysis using analysis of variance (ANOVA) on ranks showed that the dual-cured composite light polymerized 1 minute after placement exhibited the lowest microleakage (P < .05) in both bonding agent groups. Conclusion: Delayed, rather than immediate, light polymerization of the dual-cured composite base reduced microleakage in Class 2 open-sandwich restorations. (Quintessence Int 2009;40:471–477)