Purpose: To evaluate the clinical performance of endodontically treated (ET) teeth restored with two different marginal forms of lithium disilicate glass-ceramic (LDG ceramic) onlays. Materials and Methods: A total of 120 posterior teeth were prepared for onlays after ET. The occlusal surfaces of the teeth were removed 1.5 to 2 mm anatomically. Different marginal forms were employed depending on the thickness of the residual axis wall of each tooth: In Group S, a 1-mm shoulder was prepared when the remaining axis wall was > 2 mm (n = 50), and in Group B, a 20- to 30-degree bevel was prepared when the remaining axis wall was ≤ 2 mm (n = 70). The access of the pulp chamber was filled with resin, and 1.5 to 2 mm of the box morphology of the pulp chamber was preserved. The pressed LDG ceramic was applied to the fabricated onlays. The teeth and restorations were checked after 6, 12, and 24 months. All available restored teeth were assessed using modified United States Public Health Service criteria. Results: During the observation period, no tooth fracture occurred. Neither tooth cracks nor secondary caries were observed in Group S or Group B. The 2-year survival rate of the teeth was 100%. Five fractured onlays were observed in Group S in the molar region, while all onlays in Group B were intact (P .05). The survival rate of the onlays was 95.83%. Conclusion: The results demonstrate that LDG ceramic onlays can preserve ET posterior teeth with a highly satisfactory outcome, though the marginal form may affect the survival rate of the onlay.