To determine the influence of root canal treatment on pain prevalence and severity and estimate the prevalence and severity of pretreatment, treatment, and posttreatment pain in patients receiving nonsurgical root canal treatment. The University of California, Los Angeles, Section of Endodontics. Systematic review with meta-analysis of data. Level 2: Limited-quality, patient-oriented evidence. Grade B: Inconsistent or limited-quality patient-oriented evidence.