Objective: The aim of the present study was to evaluate and compare the elution of monomers from three different core build-up composite materials and correlate it with the degree of conversion. Methods: Three different core build-up composite materials (a chemically cured, a photo-cured, and a dual-cured) were tested. Ten samples (diameter: 4.5 mm and thickness: 2 mm) of each material were fabricated to evaluate the release of monomers. The photo-cured samples were polymerized for 40 s and the dual-cured samples for 20 s. The samples remained undisturbed for 10 min and then were stored in 1 ml of 75% ethanol at room temperature, and the storage medium was renewed after 24 h, 7 and 28 days. From the storage medium that was removed, samples were prepared and analyzed by LC-MS/MS. Additionally, four samples of each material were tested for the degree of conversion by using a FT-IR spectrometer. Results: The three composite materials differed significantly concerning the elution of monomers (BisGMA: p <0.0001; TEGDMA: p <0.0001; and Bisphenol A: p <0.0001). A significantly higher amount of BisGMA and TEGDMA was released from the chemically cured composite compared to the other two materials. Between the photo-cured and the dual-cured material the latter eluted significantly higher amounts of BisGMA and TEGDMA. During the storage of the samples, the amounts of the eluted substances decreased. The degree of conversion of the chemically cured composite was significantly lower compared to the other two materials. Significance: Using the present parameters, the photo-cured material released less monomer and therefore they might be less dangerous with respect to toxicological effects. © 2009 Academy of Dental Materials.