To evaluate the effect of a dentine bonding system to sclerotic dentine in comparison with normal dentine.The efficacy of the dentine bonding system to sclerotic dentine was examined by measuring wall-to-wall polymerization contraction gap width. The dentine cavity wall was pretreated with an experimental dentine bonding system with and without a dentine primer. The dentine primer was glyceryl mono-methacrylate (Blemmer GLM, NOF Corp., Tokyo, Japan) (GM), which contained esterified methacrylate with a polyvalent alcohol, which is similar to 2-HEMA. The structure of sclerotic dentine and the changes to that structure caused by etching were observed using a scanning electron microscope (SEM).With GM priming, complete marginal integrity was obtained regardless of the type of dentine. Without GM priming, complete marginal integrity was obtained in half of the specimens of the sclerotic dentine, and was not obtained in any of the specimens of normal dentine. In the SEM study, the structure of sclerotic dentine was considered to be viable for adhesion. However, this was not the case when etched with phosphoric acid.It was concluded that sclerotic dentine had a clear advantage over normal dentine with regard to the adaptation of resin composites. Therefore the structure of sclerotic dentine possesses a naturally derived structure to which a primer may attach. Sclerotic dentine is part of the body's natural defenses and should be preserved. It should not be exposed to acid etching which would damage its structure.