en Oral & maxillofacial surgery Oral & maxillofacial surgery Original article Original article <div style="text-align: justify;"><strong>Background and Aim</strong>: Discolored teeth are among the most common reasons behind seeking esthetic dental treatments. Resin infiltration is a new technique for treatment of white spot lesions (WSLs). In this technique, 15% hydrochloric acid (HCl) is used to cause surface erosion. The aim of this study was to compare the enamel erosion caused by 15% and 18% HCl gels after different application times.<br> <strong>Materials and Methods: </strong>In this experimental study, sixty sound enamel surfaces were randomly divided into six groups of 10 (G1-G6). G1 specimens were subjected to 15% HCl gel for 75 seconds. G2 and G3 samples were subjected to 15% HCl for 90 and 120 seconds, respectively. G4, G5, and G6 specimens were subjected to 18% HCl for 75, 90, and 120 seconds, respectively. The specimens were then evaluated under a scanning electron microscope (SEM), and the depth of erosion was recorded. Data were analyzed by using one-way analysis of variance (ANOVA) and Tukey&rsquo;s post-hoc test with a 95% confidence interval.<br> <strong>Results: </strong>The depth of erosion in 18% HCl groups was greater than that in 15% HCl groups after the same application times (P<0.05). The depth of erosion was not significantly different between G3 and G4 samples (P>0.05).<br> <strong>Conclusion: </strong>The results of this study showed that the application of 18% HCl for 75 seconds is a suitable alternative to the application of 15% HCl for 120 seconds in the resin infiltration technique.</div> رزین اینفیلتریشن,HCL15%,HCL 18%,عمق اروژن,SEM Enamel microabrasion, Hydrochloric acid, Tooth erosion, Scanning electron microscopy 14 19 http://jrdms.dentaliau.ac.ir/browse.php?a_code=A-10-598-1&slc_lang=en&sid=1 M MalekiGorji mohsen_malekigorji@yahoo.com 100319475328460010658 100319475328460010658 Yes Postgraduate Student, Oral & Maxillofacial surgery Dept, dental school, S Banava sbanava@gmail.com 100319475328460010659 100319475328460010659 No Assistant Professor, Department of operative & Restorative Dentistry and Dental Materials, MA Saghiri Saghiri2009@gmail.com 100319475328460010660 100319475328460010660 No Assistant Professor, Department of Ophthalmology & Visual Sciences,