In recent years, the elution of monomers from dental materials has been a cause for public concern. Urethane dimethacrylate, commonly abbreviated to UDMA, is one of the monomers that are most often tested with regard to elution from and cytotoxicity of resin-based materials. Although each chemical name represents the chemical type, chemical structure, and molecular weight of a molecule, it does not seem to be the same with UDMA. In the present paper, the different forms of UDMA are presented. These include those used by dental manufacturers to produce composite materials and the different types of urethane dimethacrylate used in studies concerning the elution of monomers from composite materials. High-performance liquid chromatography (HPLC) is usually used to detect the eluted monomers, but it does not appear to be adequate in determining the different forms of UDMA. The combination of HPLC with mass spectrometry is shown to be able to specifically identify the compounds eluted in addition to those compounds used as standards in the various studies. The fact that the same name is given to different molecules causes confusion about the results of studies testing the elusion of monomers from composite materials and their possible toxicity. © 2009 Wiley Periodicals, Inc.