This in vitro study compared the relative radiopacities of three commercially available resin-modified glass-ionomer cements (Vitremer, Fuji II LC, and Photac-Fil), an experimental resin-modified glass-ionomer (V-66), two conventional glass-ionomers (ChemFil and Fuji Cap II), and amalgam (as the control). Radiopacity was assessed densitometrically and expressed as equivalent thicknesses of aluminum. All the glass-ionomer cements were more radiopaque than enamel and dentin, with the exception of ChemFil and Photac-Fil. Apart from the control material, the experimental resin-modified glass-ionomer material, V-66, had the highest radiopacity of all the materials tested. Of the three resin-modified glass-ionomer materials tested, Fuji II LC was the most radiopaque and Photac-Fil the least. For the radiopacity of restorative glass-ionomer materials to exceed that of enamel, it should be greater than 1.5 mm of equivalent thickness of aluminum.