Objective: The aim of this randomized controlled clinical trial was to investigate the antimicrobial efficacy of traditional positive pressure (PP) irrigation compared with apical negative pressure (ANP) in tapered as well as non-tapered root canal preparation techniques. Method and Materials: Patients referred for treatment of mandibular permanent molars with necrotic pulps and apical periodontitis were considered for this study. A total of 32 eligible teeth were evaluated based on strict inclusion and exclusion criteria and randomly assigned as follows: Group 1: LightSpeed LSX 0.02 preparation and A NP irrigation (n = 8). Group 2: LightSpeed LSX 0.02 preparation and PP irrigation (n = 8). Group 3: ProTaper preparation and A NP irrigation (n = 8). Group 4: ProTaper preparation and PP irrigation (n = 8). A ll teeth were sampled immediately upon access (S1), after completion of the chemo- mechanical preparation (S2), and one week later after intracanal medication (S3). All samples were microbiologically processed immediately and plated in duplicates on aerobic plates, anaerobic plates, and chocolate plates and colonies statistically analyzed. Results: No statistically significant differences were found between groups at S1. When comparing colony-forming unit (CFU) levels at S1 and S2 within groups, the reduction was statistically significant in all groups. While no significant difference was found in the proportion of teeth that culture positive at either S2 or S3 by shaping groups, a significant difference was noted when comparing PP irrigation and A NP groups. The percentage of samples in the PP irrigation group with a negative culture at both time points was 67%, compared to 100% among the A NP group. Conclusion: Within the limitations of the present study, taper and apical size failed to demonstrate a difference in microbiological reduction of cultivable bacteria. On the other hand, A NP revealed a significant difference when compared to PP irrigation. Schlagwörter: apical negative pressure, apical periodontitis, irrigation, root canal disinfection