Glass-ionomer cements, with their ability to bond to dentin, have been reported to produce less microleakage than several other restorative resin materials. This matched-pair study reports the effect on microleakage of placement of a light-curing glass-ionomer cement used as a liner when extended out to the dentinal cervical cavosurface margin or held short of that margin. Pairs of circular Class V cavities involving 50% enamel, with bevel, and 50% cementum were cut in 31 extracted third molars. After placement of the glass-ionomer cement, etching of the bevel, and placement of a resin bonding system, microfilled resin was placed over the bonding agent in one increment, light-cured, finished, and given 500 thermocycled 500 times between 5 and 55 degrees C. A silver nitrate staining technique was used to evaluate microleakage measured in graticular units (gu) along the dentinal interface under stereomicroscopy. The extension of the glass-ionomer liner out to the dentinal cavosurface margin did not significantly improve microleakage performance compared to performance of the liner when it was placed short of that margin and covered by a resin veneer. From other considerations, such as the potential for mechanical breakdown of the glass-ionomer cement when exposed to the oral environment and abrasion, and the possibility for improved esthetics, the clinician should consider holding the glass-ionomer liner short of the dentinal cavosurface margin and covering it with a composite resin veneer of a thickness consistent with structural integrity.