Purpose: To evaluate the frequency, distribution and quality of root canal treatment in an adult Nigerian subpopulation and to determine the prevalence of apical periodontitis. Materials and Methods: The periapical status and length of root fillings of 756 patients attending Obafemi Awolowo University Teaching Hospital, Ile-Ife, Nigeria for the first time were evaluated using full mouth periapical radiographs. The length of the root canal filling was considered 'adequate' if it was ≤ 2 mm from the radiographic apex, 'underfilled' if it was > 2 mm short of the apex, and 'overfilled' if filling material extended beyond the radiographic apex. Periapical status was assessed using the periapical index (PAI) with teeth classified as having apical periodontitis if the score was over 2. Results: Overall, 61.2% of the patients had root-filled teeth and 67.2% featured apical periodontitis. Of the 21,468 teeth examined, 12.2% had been root filled, and of these 41% exhibited apical periodontitis. The prevalence of rootfilled teeth was higher in the younger patients, while the prevalence of apical periodontitis in root-filled teeth was similar between age groups. Root-treated teeth that were overfilled or were mandibular incisors had the highest prevalence of apical disease. Overfilled teeth were more prone to developing an apical radiolucency than were underfilled teeth (P 0.001 and P 0.05, respectively). Conclusions: The prevalences of endodontically treated teeth and apical periodontitis were within the range reported for other countries; however, a very large number of patients required treatment or retreatment. Keywords: apical periodontitis, root canal treatment