Marginal leakage in amalgam restorations often precedes the development of secondary caries. One potential way to improve the marginal seal of such restorations, and thus minimize the risk of caries development, is to apply a glass-ionomer base prior to amalgam placement. This study compared microleakage resistance among amalgam restorations placed with and without light-cured glass-ionomer base materials. Preparations were made in extracted human molar teeth. Four groups were studied, including copal varnish, which was used as a control. All specimens were restored with amalgam. Results indicated significantly greater leakage at the cavity wall/base interface for restorations with a copal varnish cavity liner than for those with a glass-ionomer base. Leakage differences among bases were also found at the amalgam/base interface. Best results were obtained with a dual-cure resin-based system. These samples showed minimal leakage at both the cavity wall/base and the base/amalgam interfaces. These findings suggest that light-cured glass-ionomer bases can be effective in the prevention of microleakage in amalgam restorations.