Purpose: The aim of this study was to examine the effect of remaining buccal cavity wall, remaining cervical tissue, and post on the fracture strength of endodontically treated restored premolars. Materials and Methods: Teeth were randomly allocated to 10 experimental groups (n = 10) according to cavity design and presence or absence of post or to a control group. After thermal and mechanical aging, ramped loading until fracture was performed. Results: A high cervical outline (417 N) and the presence of a post (189 N) increased fracture strength, but both factors together had an antagonistic effect of −218 N, resulting in a higher strength of not 606 N (417 + 189) but 388 N. The risk of catastrophic failure increased (OR = 3.17) when a post was present.