The use of cavity disinfectants with composite resins has raised important questions regarding their potential adverse effects on bond strength. The purpose of this study was to evaluate the effect of cavity disinfection on the bond strength of composite resin to dentin. Buccal surfaces of 64 caries-free extracted human third molars were ground flat by diamond flat-end cylinder bur, polished with a series of silicon carbide abrasive papers and mounted in autopolymerizing acrylic resin. They were randomly distributed into four groups (of 16). In group 1 (control group), dentin surfaces were treated with 35% phosphoric acid, Permagen primer and Permagen bonding resin and no disinfectant was used. In group 2, cavity disinfectant was applied and acid etching, priming and bonding procedures were performed as in group 1. In group 3, cavity disinfectant was applied after acid etching. In group 4, cavity disinfectant was applied as in group 3, but it was rinsed off before priming and bonding procedures. Then a Teflon mould 3 mm thick and 4 mm in diameter was attached to the dentin surfaces, filled with composite resin and light polymerized. After the specimens had been thermocycled, shear bond strengths were determined. Application of the disinfectant before or after acid etching significantly reduced the shear bond strength of composite to dentin (P < 0.05). Rinsing off the cavity disinfectant before the bonding procedure did not affect the bond strength (P > 0.05). The use of cavity disinfectant with composite resin restorations can influence the dentin bond strength.