Statement of problem. Enhancement of bond strength between new and old composite usually requires increasing the surface roughness to promote mechanical interlocking and coating of old composite with unfilled resin bonding agents to advance surface wetting and chemical bonding. Purpose. The purpose of this study was to evaluate the effect of combinations of surface treatments and bonding agents on the shear bond strength between new and old composite. Material and methods. Six surface treatments, two bonding agents, and an untreated control comprised 18 different subgroups. Results. The use of unfilled resin, alone or combined with silane, was the most effective procedure to enhance the shear bond strength of the repaired composite specimens, irrespective of the surface pretreatment processes. Silanation and unfilled resin slightly but not significantly improved the repair strength compared with unfilled resin alone. Conclusions. Different combinations of surface treatments and bonding agents affect shear bond strength differently. The highest shear bond strength values were achieved by grinding the surface with green Carborundum stone or sandblasting, whereas the lowest values were obtained with hydrofluoric acid as the surface treatment agent. (J Prosthet Dent 1997;77:122-26.).