The objectives of this study were to determine the shear bond strength of a nanohybrid resin-based composite (RBC) when repaired with three different RBCs and to test the hypothesis that the type of composite used to repair, aging, and surface treatment prior to bonding affect shear bond strength on repairs of nanohybrid RBCs. One hundred fifty cylinder specimens were built using a nanohybrid RBC (base cylinder). Half of these specimens were aged in distilled water for 24 hours at 37 degrees C, at which point the surface was roughened with abrasive discs, acid-etched, silanized, and adhesive-treated. The remaining 75 specimens were not aged and did not receive surface treatment. Over each base cylinder, both aged and nonaged, a new cylinder was built (repair cylinder) from either nanohybrid RBC (n = 25), microhybrid RBC (n = 25), or microfilled RBC (n = 25). All specimens were aged in distilled water for 24 hours at 37 degrees C and a shear bond strength test was performed. Results indicated that aging and surface treatment significantly affected the shear bond strength of repaired samples (P < 0.0001). All aged groups showed lower shear bond strengths than their matching nonaged group. The type of RBC also affected the shear bond strength (P < 0.0001). Repair with either a nanohybrid RBC or a microhybrid RBC demonstrated the highest shear bond strength, while the lowest shear bond strength was observed when the cylinder was repaired with a microfilled RBC.