Introduction: There is a broad consensus among the studies that the presence of Apical Periodontitis (AP) at the time of endodontic treatment (ET) has a negative influence on prognosis, which is an important issue to consider. Objectives: The aim of this investigation is to estimate the prevalence of ET, the prevalence of AP in the adult population and the index of decayed, missing and filled teeth (DMFT) attending to the Dental School of Faculty of Medicine on the University of Coimbra. Methods: Panoramic radiographs of all 229 patiens attending the Dental School of the Coimbra Medical University during the period of 1st January to 31st December of 2011 and periapical x-ray were registered to root-filled teeth. Statistical analysis was performed by SPSS, version 18 and the statistical tests were evaluated with the significance level of 5%. Results: The sample size was 229 patients, distribute by gender, 39.7% were men and 60.3% were women aged between 18 and 84years. The prevalence of ET in the sample was 31% and the prevalence of AP in the patient's was 28.9%. A total of 4610 teeth were examined, based on that the prevalence of AP was 3.3% and the prevalence of AP in endodontically treated teeth was 3.5%. However, when endodontic treated teeth were examined, the prevalence of AP was 39.8%. In rootfilled teeth the prevalence of AP was 53.4%. When investigated from the point of view of the DMFT index, differences on gender were not statistically significant and index showed an increase through life due to the cumulative effect of caries lesions. Conclusion: The prevalence of AP in the patient's sample was similar to the previous studies reported in Europe, specifically Ireland and Norway. Furthermore, the results resembled to the study in Portugal at 1998. Keywords: cross-sectional study, apical periodontitis