Temporary filling materials are used in endodontics to prevent contamination of the root canal system during intertreatment visits. Recently, a new composition resin (TERM) has been introduced as an interim restoration in endodontics. The purpose of this investigation was to evaluate the efficacy of this resin as an interim restoration and possible effect of access cavity design with respect to its sealing ability compared with other temporary restorative materials. Sixty human teeth were divided into two categories, and two types of access cavities were prepared, namely tapered access with walls divergent toward the occlusal surface and straight access with parallel walls. The prepared teeth from each category were divided into three subgroups of ten each to receive Cavit (group I), IRM (group II), or TERM (group III). After the fillings were placed, the teeth were transferred into a humidor where they stayed for 24 hours. The teeth were then thermocycled. Each group was thermocycled for 2 hours at 1 minute intervals at 60 degrees C and 4 degrees C. Then the teeth were sealed with a compound and coated with two layers of clear varnish, with the exception of cavity margins. Each occlusal surface was immersed in a vial containing silver nitrate. The teeth were sectioned longitudinally, and the degree of dye penetration was evaluated under a dissection microscope according to set criteria. Our findings demonstrated that design of cavity preparation was not a functional parameter in leakage pattern. Results indicated that Cavit had the best sealing ability whereas IRM showed the maximum dye penetration.