The aim of this study was to investigate the post-treatment apical periodontitis in endodontically treated teeth related to the technical quality of root fillings and restorations in Thai population. Full-mouth periapical radiographs from Thai patients attending the Faculty of Dentistry, Mahidol University, Thailand were examined. The periapical status, quality of root fillings and coronal restorations, existence of intraradicular post and its relationship to the root fillings were evaluated. Among 1586 patients, 37.4% presented endodontically treated teeth. The total number of endodontically treated teeth was 1511, and 35% presented apical periodontitis. Combined data for root filling and coronal restoration qualities revealed significantly better outcome in teeth with adequate qualities of both parameters than the other combinations (P < 0.001). The prevalence of post-treatment apical periodontitis among Thai population was relatively high and significantly associated with substandard technical quality of root fillings and restorations.