Objectives: There are no data comparing coronal leakage of teeth prepared to different apical sizes and obturated with gutta-percha and sealer. The aim of this study was to investigate the effect of apical preparation size on the leakage of obturated root canals. Large apical openings are encountered as a result of pulp necrosis in immature teeth, apical resorption, or over-enlargement of the apical foramen. Complete cleaning, shaping, obturation, and apical seal of root canal systems are essential for the success of root canal treatment. Method and Materials: One hundred twenty-five extracted human teeth were divided into groups containing 25 samples each and prepared to apical file sizes 30, 40, 50, 60, and 70. Twenty teeth served as positive and negative controls. Obturation was completed with gutta-percha and sealer via warm vertical compaction. Bacterial leakage was investigated after 112 days using Proteus vulgaris. Data were analyzed via independent-samples Kruskal-Wallis test. Results: The average time for leakage of apical preparation sizes 30, 40, 50, 60, and 70 were 57.5, 52.4, 47.2, 37.5, and 28.4 days, respectively. Significant differences in leakage were observed between apical preparation sizes 70 versus 30, 70 versus 40, 70 versus 50, as well as 60 versus 30. A trend for more leakage occurred when apical preparation sizes exceeded size 60. Conclusions: Based on these results, it appears leakage of gutta-percha and sealer as obturation materials increases when apical preparation size exceeds 60. Consideration should be given to using sealing materials other than gutta-percha and sealer when the apex size exceeds 60. Keywords: bacterial leakage, gutta-percha, microleakage, open apex, Proteus vulgaris, trauma