During root canal therapy cotton may be placed in the pulp chamber before placement of a temporary filling. This study evaluated the effect on the sealing of a zinc oxide-calcium sulfate-based temporary filling material when fibers of the internal cotton passed through the filling material and reached the external surface of the temporary filling. Glass tubes were filled with a 3.5 mm thickness of the temporary filling incorporating a bundle of cotton fibers that passed from the inner surface of the restoration to the outer surface. The time for methylene blue stain to penetrate the filling was compared among four groups. Group 1 (large bundles of fibers) had 20 to 40 fibers, group 2 (medium bundles) had 10 to 15 fibers, group 3 (small bundles) had 3 to 5 fibers, and group 4 (negative controls) had no cotton fibers. All samples with cotton fibers leaked within 12 min. None of the negative controls leaked within the 21 -day observation time. These results demonstrate that even a very small amount of cotton trapped between the wall of the tube and the filling material dramatically reduced the sealing quality of the temporary restoration.