Dental caries is one of the most prominent chronic diseases worldwideand the detection of caries lesions is still a matter of investigation in dentistry.•Radiography is a very suitable manner of diagnosing proximal carious lesions.Recent studies have focused on the dental application of 3D imaging modalities to overcome the superimposition of 3D anatomic structures in 2D images.•But presence of metallic objects in the oral cavity compromise the results of CBCT.•The current study aims to detect the accuracy of Cone Beam Computed Tomography in detection of proximal caries in lower posterior teeth adjacent to amalgam restored teeth.•The Study included 115 mandibular natural posterior permanent teeth (whether premolars or molars) which were sound or with proximal caries of different degrees and without buccal or lingual extension of caries.•Each tooth was placed in acrylic cast filled with impression compound adjacent to class 2amalgam restored tooth in a position simulating normal anatomical position. The cast was surrounded by pink wax to simulate soft tissue. •Teeth were imaged using Planmeca ProMax 3D Mid CBCT machine (Asentajankatu, Helsinki, Finland).•Images were assessed in the sagittal, axial and coronal planes to detect proximal caries and results were made on a 2 points scale (yes= caries present/ no= caries absent).•Results were made for CBCT and for each orthogonal plane individually. Teeth were sectioned mesiodistally and imaged using stereomicroscope magnification (40 X).•The readings were made for each sectioned tooth on a 2 points scale "caries present, caries absent".•Results of CBCT were compared to stereomicroscope as a gold standard.•Results showed high sensitivity (92.6%), low specificity (16.7%) and moderate diagnostic accuracy of CBCT (52.6%). Also sagittal plane showed higher sensitivity (100%), lower specificity (6.7%) and moderate diagnostic accuracy (50.9%). While the coronal and axial planes showed the same results in almost all tests.