Objectives: The present study examines the influence of cavity type, and size of composite restorations on cuspal flexure due to polymerization shrinkage. Methods: Thirty healthy premolars were selected and divided into two experimental groups. Group 1: Mesial, mesiodistal, and MOD cavities were successively prepared in each tooth by means of the SONICSYS approx system (KaVo ®) using a nº 2 (small) diamond tip. Group 2: The same cavity preparation sequence was followed but a nº 3 (medium) tip was used. Cavity preparations were performed and afterwards restored using the same adhesive system and resin composite. Cuspal displacement was measured 15 min after completion of each type of restoration. Results: Under the experimental conditions used, polymerization shrinkage of composite restorations resulted in an inward deflection of the cusps in all the situations evaluated. The Friedman global test found significant differences according to the cavity type and size (p<0.05). The Wilcoxon test revealed that deflection was significantly affected by the amount of dental tissue lost, since practically no movement was observed in the case of mesial cavities but extensive displacements were recorded for MOD restorations (p<0.05). However, no influence of bucco-lingual width on cuspal flexure was detected by the Student?s t test (p>0.05). Conclusions: The present study demonstrates that significantly higher cuspal deflection is observed in MOD restorations, showing that the degree of dental removal influences the cuspal flexure from polymerization shrinkage of composite restorations.