Apical resection is fundamental in obtaining a correct apical seal. The purpose of this study was to evaluate how the apical root resection angle and the cavity made by ultrasonic retrotips may influence the apical seal. A total of 48 extracted teeth were endodontically treated and sealed by guttapercha vertical compaction: 24 were resected with a 45 degrees angle and 24 with a 90 degrees one. An ultrasound source (P.M. 400 EMS) and a CT5 Scaler were used to make the retrograde cavity that was filled afterwards with EBA-zinc oxide-eugenol alumina-added cement. Apical leakage was determined using fuchsin and assessed after the roots were sectioned longitudinally. Linear dye penetration in dentin and at the interface between dentin and cement was measured with a stereomicroscope (x 12 magnification), and the results were statistically analyzed. Results showed that there was less infiltration both in dentin and in the space between the filling and the dentinal wall in the group with the 90 degrees angle, but this difference was statistically significant (p < 0.0001) only for the dentin. None of the samples showed leakage greater than the depth of the preparation. An apical cavity of 3 mm or more along the vertical axis can produce a safe and effective seal.