Objective: To compare the fracture resistance of mandibular premolars restored with mesial-occlusal composites after access with traditional endodontic cavity (TEC) or contracted endodontic cavity (CEC) designs. Materials and Methods: Forty-five virgin single-rooted mandibular premolars were randomly divided into three groups and embedded in acrylic resin moulds. Group I (n = 15) remained untreated. Group II and III were prepared with mesial-occlusal cavities and accessed with TEC or CEC designs. Chambers were irrigated with 5.25% sodium hypochlorite/17% EDTA and dried. After etching and priming with Optibond, access cavities were restored with 3M Filtek. All samples were then fixed in a universal testing machine with the long axis of the roots positioned 45o to a load applied at a crosshead speed of 0.5 mm/min until failure. All specimens were visually assessed for failure modes. Fracture force data was statistically analysed by one-way ANOVA and Tukey post-hoc (P = 0.05). Results: The average fracture force for untreated teeth was 609.7 ± 279.1 N; 601.7 ± 307.9 N for mesial-occlusal cavities with TEC; and 600.9 ± 360.3N for teeth with CEC. There was no significant difference in fracture resistance between groups (F (2,42) = 3.299, P = 0.996). There was no significant difference in fracture resistance between traditional and modified groups (P = 0.994). All three groups had an equal number of "favourable" (repairable) and "unfavourable" failures (7:8), defined as irreparable failures or root fractures below the level of simulated bone. Conclusions: Modifying access outline to a contracted design did not improve fracture resistance of mandibular premolars restored with mesial-occlusal composites. Clinical Significance: A mandibular premolar with minimal caries that has been accessed endodontically with a CEC or TEC design may be restored provisionally with a class II composite with an equal measure of confidence with regard to fracture resistance. Schlagwörter: Endodontic access cavity, fracture resistance, mandibular premolars