['The aim of this clinical study was to assess the influence of irrigation needle gauge on endodontic irrigation flow rates.', 'In vivo assessment.', 'Five specialist endodontists performed intracanal irrigation procedures on 50 mesiobuccal canal of mandibular first molars using three different irrigation needle gauges. Data of time taken for irrigation was recorded by an irrigation testing system and analyzed using independent sample "T" test and one-way analysis of variance (ANOVA) test. The level of significance was set at P < 0.05.', 'The following tests were used for the statistical analysis: Independent sample "T" test, one-way ANOVA test, and post hoc multiple comparison was carried out using Tukey\'s honest significant difference (HSD) test using Statistical Package for the Social Sciences (SPSS) version 16 for Windows.', 'The average flow rate of 26 gauge was 0.27 mLs(-1), of 27 gauge was 0.19 mLs(-1), and of 30 gauge was 0.09 mls(-1). There was statistical significance among the gauges (P < 0.001). 26 gauge had highest flow rate when compared with other groups followed by 27 gauge and 30 gauge respectively. The operator variability for flow rate of three endodontic irrigation needle gauges (26 gauge, 27 gauge, and 30 gauge) was found to be not significant.'] Does an in vivo assessment of the influence of needle gauge on endodontic irrigation flow rate?