Objectives:The emergence of Mineral Trioxide Aggregate (MTA) as a root-end filling material has generated a lot of interest due to its superior sealing ability and biocompatibility. Previous studies have shown Portland cement to be non-toxic, similar in composition and physical properties to MTA. The purpose of this study was to compare the in vitro sealing ability of MTA and Portland cement when used as root end filling materials. Methods:Root canals of twenty-four freshly extracted single-rooted human premolar teeth were prepared and obturated using standard techniques and retrofilled with each of the test materials: ProRoot MTA (Tooth colored formula) (WMTA), ProRoot MTA (PMTA), White Portland (WP) and Ordinary Portland cement (OP). Two teeth served as positive and negative controls respectively. The prepared teeth were immersed in methylene blue dye for seventy-two hours and then assessed for dye leakage by longitudinally splitting the teeth. The depth of dye penetration was measured and expressed as a percentage of the length of the retrofilling. Data was analyzed using ANOVA and Fisher's LSD (p<0.05).Results:The mean depth of dye penetration was 62.72% for WMTA, 54.25% for PMTA, 62.06% for WP and 53.80% for OP cement. WMTA showed significantly greater dye penetration than both PMTA and OP cement while WP showed significantly greater penetration than PMTA and OP cement. There was no significant difference between PMTA and OP and between WMTA and WP cements. None of the teeth in any of the groups showed leakage beyond the retrofillings. Although the materials varied in the depth of penetration of the dye, all the four materials tested effectively sealed the root canals. Conclusions:Given the low cost and apparently similar sealing ability of MTA and Portland cement, it is reasonable to consider Portland cement as a possible substitute for MTA although further studies need to be carried out.