Treatment of dentin with acids results in a surface change that is potentially useful for mechanical retention of resins in tooth structure. This study examined the effect of etching of dentin on the seal of cavities restored with composite resin. Eighty cavities, each measuring 2.0 mm in diameter and 1.5 mm in depth, were prepared in dentin of extracted human teeth and randomly assigned to two equal groups. The experimental group was etched with 37% phosphoric acid for 15 seconds before restoration. The control specimens were similarly restored but without any etching of dentin. After the specimens were thermocycled, the seal of each cavity was assessed by measurement of the gap between the restorative material and the tooth structure. The experimental group registered significantly better adaptation to the cavities than did the control group.