Althouh cone beam computed tomography (CBCT) is popular for dental implant planning, the horizontal mesiodistal space of the edentulous ridge is still conventionally measured with a handheld sliding caliper in the oral cavity or on a plaster cast. For clinical application in implant planning, our aim was to evaluate the accuracy of CBCT horizontal mesiodistal linear measurements in comparison with conventionally obtained direct measurements on plaster casts. Postoperative CBCT acquisitions and plaster casts of 27 patients with adjacent posterior mandibular implants were analyzed in a prospective clinical study. On CBCT images, two observers assessed the inter-implant distances on axial and sagittal views. On plaster casts, the inter-implant distances were measured with a digital caliper. CBCT measurements on axial and sagittal views were, on average, 0.2 mm larger than measurements on plaster casts. Correlation was perfect between measurements of the same inter-implant distance performed by the observers on CBCT images and on plaster casts. When compared with conventionally obtained direct measurements on plaster casts, CBCT views slightly overestimated (mean 0.2 mm) the horizontal mesiodistal measurements between two implants as reference points. CBCT imaging is sufficiently accurate to evaluate mesiodistal distances on axial and sagittal views for dental implant planning in clinical practice.