SUMMARY: The knowledge of the anatomical relationships between the floor of the maxillary sinus and the apices of the posterior maxillary teeth is very important when planning an endodontic therapy or surgeries in the area. The objective of this study is to establish the distance between the apex of the palatal root of the first maxillary molar and the floor of the maxillary sinus, in a sample of 61 cone-beam computed tomography. A cross-sectional observational study was carried out. We proceeded to measure the distance between the apex of the palatal root of the first upper molar and the floor of the maxillary sinus in 61 cone-beam computed tomography, obtaining measurements of a total of 107 first maxillary molar palatal roots. The images were stored in DICOM format and analyzed with the Sky View software and the Stata 11.0. for statistical analysis. The average distance recorded between the apex of the palatal root of the first upper molar and the floor of the maxillary sinus was 1.2 mm. The greatest distance was 12.5 mm and the smallest was 0 mm. The measurement of 0 mm was presented in 48 cases and reached 44.9 % of the sample. It is common for radicular apices to be located within the maxillary sinus. The clinician must know this condition to prevent accidents and complications in the endodontic therapies as much as possible.