Introduction: Previous studies have shown the difficulty in removing calcium hydroxide (Ca[OH](2)) paste from the root canals before root filling. Methods: Mesial and distal canals of 30 mandibular molars were prepared with the Wave One Primary (25/.08) and Large file (40/.08) (Dentsply Tulsa Dental Specialties, Tulsa, OK), respectively. All canals were then filled with Ca(OH)(2). The teeth were divided into the following 3 treatment groups (each with n = 10): (1) instrumentation with needle irrigation, (2) instrumentation with irrigation and passive ultrasonic activation (PUI), and (3) the GentleWave system (Sonendo, Inc, Laguna Hills, CA) without instrumentation. The irrigation time in each group was 7.5 minutes. To further test the efficiency of the GentleWave system, shorter times of 90 seconds were tested using water alone. Reconstructed micro computed tomographic scans were used to measure the volume of the canals and Ca(OH)(2) after instrumentation, initial filling of Ca(OH)(2), and after its removal. The percentage of Ca(OH)(2) remaining in the canals was calculated. Results: None of the 10 teeth (30 canals) in the conventional irrigation and PUI groups were completely cleaned of Ca(OH)(2) in 7.5 minutes. In the apical third of mesial and distal canals, respectively, conventional irrigation removed 47.82% 16.36% and 77.68% 12.82%, PUI removed 61.66% 25.54% and 88.85 12.06%, and the G.entleWave system removed significantly more Ca(OH)(2) (P < .05) with 100% and 98.78% 3.84%. Additional experiments in 10 teeth, using only water as the irrigant, revealed that the GentleWave system removed 99.85% and 99.97% of Ca(OH)(2) within 90 seconds without the use of any instruments in the mesial and distal canals, respectively. Conclusions: The study confirms the difficulty to remove Ca(OH)(2) from root canals using conventional methods. The GentleWave system removed the paste within 90 seconds using water irrigation alone.