The purpose of this in vivo study was to evaluate the microleakage of indirect composite resin inlays and to determine the thickness of composite resin luting material. Twenty periodontally involved posterior teeth scheduled for extraction were selected and Class II cavities were prepared. Ten cavities had gingival margins placed in enamel above the cementoenamel junction, and the other 10 were placed in dentin below the cementoenamel junction. Six months after cementation, the teeth were extracted and analyzed. The leakage was higher in inlays with gingival margins in dentin than in inlays with gingival margins in enamel. The cement thickness was between 15 and 40 um, a clinically acceptable range.