Objective: To evaluate the effects of filler addition on the biaxial flexural strength and modulus of two commercial adhesive systems. Method and Materials: The systems were either unfilled (Adper Single Bond [3M ESPE] and Prime and Bond 2.1 [Dentsply]) or filled (Adper Single Bond Plus [3M ESPE] and Prime and Bond NT [Dentsply]). Resin disks of each product (0.6 mm thick and 6.2 mm in diameter) were prepared in Teflon molds (n = 11 each group; 44 in total). The adhesive solutions were light activated with a halogen light-curing unit. The disks were stored dry in the dark 24 hours before biaxial flexural testing in a universal testing machine until fracture (1.27 mm/minute; model 5844, Instron). Data were statistically analyzed by one-way ANOVA and Tukey post hoc test at a preset alpha of 0.05. Results: Flexural strength values (MPa) (SD) were Single Bond 62 (16); Single Bond Plus 46 (15); Prime and Bond 59 (13); and Prime and Bond NT 85 (16). The flexural moduli (MPa) (SD) were Single Bond 1,026 (241); Single Bond Plus 1,180 (188); Prime and Bond 744 (146); and Prime and Bond NT 1,539 (267). Based on the results, Prime and Bond NT (filled) exhibited higher flexural strength and modulus than Prime and Bond (unfilled), while no significant difference in both parameters was noted between Single Bond and Single Bond Plus. Conclusion: Filler addition to bonding agents can increase the flexural strength and modulus; however, results are product dependent. Schlagwörter: biaxial flexural test, bonding agents, flexural modulus, flexural strength, fillers