The purpose of this study was to compare the dye leakage of an MTA apical barrier after immediate placement, 1 hour, 4 hours, 24 hours, and 1 week of setting time. One hundred twenty single-rooted extracted teeth were accessed and the apical three millimeters resected. The teeth were instrumented with 0.06 tapered Brasseler EndoSequence series rotary instruments to the apex. The same instrument was then placed in the apical opening and advanced to D16, resulting in an apical opening of 1.36mm. The teeth were randomly assigned to the five test groups, and ten teeth each were placed in positive and negative control groups. Teeth were placed in flower arrangement foam, rinsed with 5.25% sodium hypochlorite, dried, and a five millimeter apical barrier of MTA was introduced to the apex. After allowing the MTA to set to the specific test time, the tooth was filled with gutta percha and Cavit. An acrylic barrier was placed over the entire tooth except for the resected root end, and each tooth was placed in methylene blue dye. They were then demineralized, dehydrated, and cleared to allow visualization of the leakage. The amount of leakage was evaluated with a measuring microscope in mm. Data were analyzed using a one-way ANOVA and Tukey's HSD test. Results indicated significantly more leakage occurred in the immediate placement and 1 hour setting time groups compared to the 4 hours, 24 hours, and 1 week setting time groups (p<0.05). While the immediate placement group exhibited more leakage (4.2mm) than the 1 hour setting time group (3.8mm), the difference was not significantly different. In addition, the difference in leakage between the 4 hour (1.7mm), 24 hour (1.7mm), and 1 week (1.6mm) setting time was not significantly different. In this in-vitro model, allowing an MTA apical barrier to set for at least 4 hours appears to be most beneficial.