Microleakage was assessed in Class V composite resin restorations, placed in extracted non-carious human premolar teeth after the enamel cavosurface margins of the preparations had been etched with phosphoric acid for 5, 15, or 30 seconds. The restored teeth were thermocycled, place d in methylene blue dye, invested, and sectioned. The extent of dye penetration along the tooth-restoration interface in the three etching groups was compared. The group etched for 5 seconds showed the greatest leakage; leakage was statistically significantly greater than that in the groups etched for 15 or 30 seconds. The tooth-restoration interface at the occlusal and gingival margins showed more leakage than did the mesial or distal margins.