Objective: To determine the effect of irrigation time on the removal of dentin debris from root canals irrigated with passive ultrasonic irrigation comparing 2 flushing methods. Method and Materials: Root canals with a standardized groove in 1 canal wall, which was filled with dentin debris, were irrigated ultrasonically or with syringe irrigation. The quantity of dentin debris after irrigation was determined. In groups 1 to 4, the irrigant was activated by an ultrasonically activated file. In groups 1 and 2 (n = 20), the canals were flushed 3 or 1.5 minutes using a continuous flow of irrigant. In groups 3 and 4 (n = 20), the irrigation time was 1 or 3 minutes, and the canals were flushed 3 times by syringe irrigation (intermittent flushing method). In group 5 (n = 20), the root canals were flushed 3 times using syringe irrigation. Results: Significantly more dentin debris was removed when the irrigant was activated by ultrasound (P = .000). Significantly less dentin debris was removed when the root canals were irrigated with a continuous flow of irrigant for 1.5 minutes (P = .005). Conclusions: Three minutes of ultrasonic irrigation with the intermittent flush technique or a continuous flow is as effective as 1 minute with the intermittent flush technique. The efficiency of the ultrasonic irrigation with a continuous flow is time dependent, 1.5 minutes being less efficient than 3 minutes. Keywords: continuous flow, flushing method, irrigant, irrigation, passive ultrasonic irrigation, root canal, time