Shrinkage of composite resin during polymerization is responsible for the formation of a microgap at cavity margins that permits the ingress of bacteria and oral fluids. This study investigated four methods of incremental insertion of composite resin into mesio-occlusodistal cavity preparations to determine the effect of each method on microleakage at cervical margins. Four groups of 10 extracted teeth were prepared with mesio-occlusodistal cavities in which one proximal box ended on enamel and the other ended on the root surface. Teeth in each group were restored with a light-curing, hybrid composite resin and its bonding agent. In group 1, the composite resin was inserted in one increment. In group 2, insertion was in two equally thick horizontal layers in the proximal box and one layer in the occlusal portion. In group 3, the composite resin was inserted in two diagonally placed layers in the proximal box and one increment in the occlusal portion. In group 4, the composite resin was inserted in the same way as in group 3 except that a glass-ionomer liner was first placed on the axial wall and gingival floor. All teeth were thermocycled, stained with silver nitrate, sectioned mesiodistally and scored for microleakage. The diagonal insertion technique in group 3 had the most leak-free margins when the proximal box ended on enamel. In all groups, cervical margins that ended on the root surface had extensive microleakage.