Objectives <p>To evaluate the effect of deep margin elevation (DME) on microleakage of 5Y-TZP ceramic crowns after thermomechanical loading.</p> Materials and methods <p>Forty extracted molars were allocated to five groups (<i>n</i> = 8). Standardized mesio-occluso-distal (MOD) tooth preprations were performed in all teeth and restored using two adhesive systems (one-bottle or two-bottle) in combination with two foundation restoration materials (auto-polymerizing or dual-polymerizing). Thirty-two molars received DME, while eight served as a control without DME. Following standardized tooth preparation, 5Y-TZP zirconia crowns were fabricated and bonded using an auto-polymerizing luting resin. While the specimens were stored in water for 150 days, they were subjected to thermomechanical loading with 37,500 thermal cycles (5&#xa0;°C/55°C) and 1,200,000 mechanical cycles at 98&#xa0;N in a chewing simulator. After loading, the specimens were immersed in fuchsine dye for 48&#xa0;h, sectioned sagittally, and evaluated for dye penetration under an optical microscope.</p> Results <p>Significant differences in microleakage were observed at the foundation restoration margins. The dual-polymerizing foundation restoration material demonstrated significantly lower microleakage than the auto-polymerizing material. The one-bottle adhesive system performed significantly better than the two-bottle system. No microleakage was detected at the crown margin in any experimental group.</p> Conclusion <p>Within the limitations of this laboratory study, the use of a dual-polymerizing foundation restoration material in combination with a one-bottle adhesive system showed favorable results for DME.</p> Clinical relevance <p>Compared with surgical crown lengthening or orthodontic extrusion, DME may reduce patient morbidity and treatment time. Clearfil DC Core, in particular, demonstrated low microleakage at DME margins, with the choice of adhesive system significantly influencing outcomes.</p>