Aim: Large apical openings are encountered as a result of pulp necrosis in immature teeth, apical resorption, or over-enlargement of the apical foramen. Cleaning, shaping, obturation and apical seal of root canal systems are essential for the success of root canal treatment. There is scarce data comparing coronal leakage of teeth prepared to different apical sizes and obturated with Mineral Trioxide Aggregate (MTA). The aim of this study was to investigate the effect of apical preparation size on the leakage of MTA obturated root canals. Materials and Methods: A total of 125 extracted human teeth were divided into five groups each containing 25 samples and prepared to apical file sizes 30, 40, 50, 60 and 70. Twenty teeth served as positive and negative controls. Obturation was completed with grey MTA. Bacterial leakage was investigated after 120 days using Proteus vulgaris. Data was analysed using Independent-Samples Kruskal-Wallis test. Results: The average time for leakage of apical preparation sizes 30, 40, 50, 60 and 70 were 58.5, 70.2, 64.8, 59.0 and 55.2 days, respectively. No significant differences in leakage were observed between apical preparation sizes 30 to 70. Conclusions: Based on the present results, it appears leakage of MTA filled root canals is largely unaffected by varied apical size. MTA appears to seal well regardless of the apical size and is considered the material of choice for obturation of larger sized apices. Keywords: proteus vulgaris, root canal obturation, silicates