Purpose: The aim of this study was to evaluate the ultrastructural characteristics and ionic profile of U937 cells after exposure to 2-hydroxyethyl methacrylate (HEMA) to shed light on the cytotoxicity of this dental adhesive and its relation to mechanisms of cell death. Materials and Methods: U937 human monoblastic cells were incubated in RPMI 1640 culture medium and exposed to HEMA at LD50. Structural changes after 5, 15, 30, 60, and 120 min were observed with transmission electron microscopy. Ionic content of Na, K, Cl, Mg, P and S was evaluated by quantitative electron probe X-ray microanalysis. Results: Our results in human monoblastic cell line U937 establish that exposure to HEMA at LD50 led to a singular mechanism of cell death characterized by changes in the morphology and ultrastructure of the cells (cell size, blebs, and organelle structure) compatible with apoptosis, but without changes in nuclear ultrastructure. These findings were consistent with our microanalytical data, which revealed a significant increase in intracellular Na and a decrease in K, along with a significant initial decrease in Cl concentration followed later (120 min) by an increase. Conclusion: All three lines of evidence (cell morphology, ultrastructural changes, and ionic profile) showed that HEMA at LD50 led to a hybrid process of cell death. We suggest that apoptosis and necrosis are part of a continuum comprising a single process of cell death. Keywords: HEMA, cell death, apoptosis Keywords: HEMA, cell death, apoptosis, necrosis, microanalysis, sodium, chlorine, potassium