Introduction: The purpose of this study was to evaluate the effect of calcium hydroxide premedication on the marginal adaptation of the mineral trioxide aggregate (MTA) apical barrier. Methods: Forty single-rooted teeth were prepared and apically resorbed using sulfuric acid for 4 days. Teeth were allocated into two groups according to whether calcium hydroxide was placed in the canals for 1 week (medicated group) or not (nonmedicated group) before placing a 4-mm MTA apical plug in the canals. The roots were mounted on aluminum stubs, the root apex was viewed from the top under scanning electron microscopy, and the maximum distance between MTA and the surrounding dentin was measured. Results: The mean gap widths in the medicated and nonmedicated groups were 70.2 μm and 130.0 μm, respectively (p < 0.05). Conclusions: Calcium hydroxide treatment improves marginal adaptation of the MTA apical plug. Copyright © 2010 American Association of Endodontists.