OBJECTIVE To evaluate the effects of hydrophilic dental resin monomers, triethylene glycol dimethacrylate (TEGDMA) and hydroxyethyl methacrylate (HEMA), on the polarization of a human monocyte cell line (THP-1). METHODS THP-1 cells were treated with resin monomers at noncytotoxic concentrations for 48 h and were analysed for CD86 and CD206 expressions using flow cytometry. The cells were stimulated for polarization in the presence of resin monomers (co-treatment) or after treatment with monomers (pre-treatment). CD86 and CD206 mRNA in co-treated cells was evaluated using quantitative real-time polymerase chain reaction. The release of TNF-α and TGF-β by pre-treated and co-treated cells was assessed using enzyme-linked immunosorbent assay. Morphological changes of macrophages during polarization were observed using bright-field microscopy. One-way analysis of variance was used for statistical analysis. RESULTS TEGDMA (1 mmol L-1 ) and HEMA (2 mmol L-1 ) did not induce CD86 and CD206 expressions in THP-1 cells but rather inhibited their expressions in the co-treated cells. The inhibitory effects also appeared at the transcription level. However, the expression of surface markers was not affected by pre-treatment with resin monomers. The release of TNF-α and TGF-β by M1- and M2-stimulated cells, respectively, was suppressed by co-treatment (P < 0.05). Microscopic studies revealed that co-treatment with resin monomers suppressed polarization-associated morphological changes such as cell volume increase. CONCLUSIONS TEGDMA and HEMA inhibited macrophage polarization to both M1 and M2 at the transcription level, and the inhibitory effects disappeared upon the removal of resin monomers from the cell culture.