This study compared the caries inhibition and microleakage of two liners under amalgam restorations. Thirty-two molars received mesio-occlusal and disto-occlusal Class II cavity preparations. Calcium hydroxide was randomly placed as a liner in one preparation of each tooth and glass-ionomer cement liner was placed in the other. Amalgam restorations were placed. Teeth were axially loaded and an acid-protective varnish was placed. Teeth were placed into artificial saliva for 3 months, thermocycled, and cycled through a caries challenge. Sections were obtained from 20 teeth and photographed under polarized light. The remaining teeth were placed in 2% basic fuchsin and sectioned. A t-test indicated that restorations with glass-ionomer cement liners exhibited significantly less demineralization and microleakage than did restorations with calcium hydroxide liners.