Resin-based methacrylate materials are widely used in restorative dentistry. They are viscous substances that are converted into solid material via polymerization. This process, however, may be incomplete, leading to the release of monomers into the oral cavity and the pulp, which can be reached through the dentin micro-channels. This opens the opportunity for the monomers to reach the bloodstream. Monomers can reach concentrations in the millimolar range, high enough to cause cellular damage, so it is justified to study their potential toxic effects. In the present work we investigated the cytotoxicity and genotoxicity of 2-hydroxyethyl methacrylate (HEMA) in human peripheral blood lymphocytes and A549 lung-tumour cells. HEMA at concentrations up to 10mM neither affected the viability of the cells nor interacted with isolated plasmid DNA during a 1h exposure. However, HEMA induced concentration-dependent DNA damage in lymphocytes, as assessed by alkaline and pH 12.1 versions of the comet assay. HEMA did not cause double-strand breaks, as assessed by the neutral version of the comet assay and pulsed-field gel electrophoresis. The use of DNA repair enzymes, spin traps and vitamin C produced results suggesting that HEMA induced oxidative modifications to DNA bases. DNA damage caused by HEMA at 10mM was removed within 120min. HEMA induced apoptosis in a concentration-dependent manner and caused cell-cycle delay at the G0/G1-checkpoint. Methylglycol chitosan displayed a protective effect against the DNA-damaging action of HEMA. The results obtained in this study suggest that HEMA induces adverse biological effects, mainly via reactive oxygen species, which can lead to DNA damage, apoptosis and cell-cycle delay. Chitosan and its derivatives can be considered as additional components of dental restoration to decrease the harmful potency of HEMA.