The relationship between the maxillary sinus (MS) and the root apices of the posterior teeth is of clinical relevance in diagnosing and treatment planning in the posterior area of the maxilla. This study aimed to assess the relationship between the MS floor and the root apices of the posterior teeth using cone-beam computed tomography (CBCT) scanning. Materials and Methods: 325 CBCT scans of patients aged 20-70 years were analyzed. Patients were divided into three age groups: young group (20-44 years), middle age group (45-59 years), and elderly group (60-70). The distance from the MS floor and the root apices of posterior teeth was measured in each group. The relationship between the MS and the posterior roots was also recorded according to Kwak classification. The results were analyzed by IBM statistic SPSS. Results: Type II was most commonly seen in the first and second molars. For premolars, Type I was often observed. The shortest distance to the floor of MS was recorded for the mesiobuccal root of the second molar and the longest distance for the palatal root of the first and second molars. No statistical differences were found between age groups (P > 0.01). Conclusion: The anatomical relationship between the MS and the root apices of the posterior teeth is of clinical importance and should be taken into consideration during surgical or endodontic treatment. © 2019 Journal of Conservative Dentistry | Published by Wolters Kluwer - Medknow.