Most dental resinous materials contain high quantities of the diluent monomer triethyleneglycol-dimethacrylate (TEGDMA). Due to its 'hydrophilic' nature, significant amounts of this substance leach into an aqueous environment, such as the oral cavity. Therefore, it is hypothesized that TEGDMA frequently interferes with oral and/or systemic tissues. In vitro studies revealed that TEGDMA is considerably cytotoxic in various cell cultures. It has also been observed that TEGDMA can easily penetrate membranes and subsequently may react with intracellular molecules. The formation of glutathione-TEGDMA adducts is of specific interest, since the nearly complete exhaustion of this molecule significantly reduces its cellular detoxifying potency. Large deletions of DNA sequences were caused by TEGDMA, resulting in high mutation frequency. In addition, TEGDMA has been identified as an important resinous sensitizer in patients and professionals. Taken together, available in vitro information, in vivo studies with animals, and clinical data as well indicate that TEGDMA may contribute considerably to local and systemic adverse effects caused by dental resins.