Few studies related the technical standard of endodontic treatment (ET) to prevalence of apical periodontitis (AP), and none assessed quality of life (QoL) as an outcome. Objectives: To determine: (1) prevalence of AP in two adult Canadian populations differing in availability of endodontists; (2) impact on prevalence of AP of root filling, restoration standard and the provider of treatment (generalist vs. endodontist); (3) impact of endodontic disease and ET on QoL. Methods: Full-mouth radiographic series/orthopantograms of 610 randomly selected first-time university patients aged 25-40 (400 in Toronto and 210 in Saskatoon) were examined by a calibrated examiner for missing teeth, presence and standard of root fillings, standard of restoration, and AP according to the Periapical Index [Ørstavik et al. Endod Dent Traumatol 1986;2:20-34]. Patients with ET were clinically examined and interviewed to determine the providers of ET and reasons for extractions. They also rated their satisfaction with ET and QoL before and after ET. Results were analyzed using chi-square and t-tests (2-tailed) and interpreted at the 5% significance level. Results: Proportion of subjects with ET was significantly higher in Toronto than in Saskatoon (39% and 25%, respectively). Presence of AP about teeth with ET (44% in Toronto, 48.5% in Saskatoon) was significantly associated with poor density (OR=2.1), short (OR=2.1) and long (OR=2.9) root fillings, and with poor radiographic quality of the restoration (OR=1.9). ET significantly improved QoL. Patients reported significantly higher satisfaction, but no significant difference in QoL, after ET provided by endodontists compared to generalists. Conclusion: Prevalence of AP about teeth with ET in the examined populations was in the range reported previously for similar age cohorts. ET significantly improved the patients’ QoL, regardless of the treatment provider.