<p>Abstract : An apical foramen is sometimes unavoidably made too wide after mechanical root canal enlargement in order to eliminate bacteria. A wide foramen may lead to unsuccessful root canal filling. In a 60-year-old female patient, dental radiography and cone beam computed tomography revealed a large periapical lesion and external resorption of the maxillary right second premolar. This case was diagnosed as chronic apical periodontitis with external root resorption, and so infected root canal treatment was performed. The buccal root canal needed enlargement to the size of #80 to remove infected dentin. Obturation of the root canal by the conventional root canal filling with gutta-percha points was difficult. On the other hand, the palatal root had a clinically closed root canal at the apical area. Long-term use of calcium hydroxide was applied to encourage the closure of the widely-formed apical foramen with deposition of hard tissue. As a result, apical barrier formation at the apical area was observed under a microscope. Root canal filling was carried out by mineral trioxide aggregate. The CAD/CAM crown was set on the tooth. At 2-year clinical and radiographic follow-up, the affected region had almost completely healed.</p>