Inflammatory injuries in the maxillary sinus may originate from root canal infections and lead to bone resorption or regeneration. This report describes the radiographic findings of 4 asymptomatic clinical cases of large reactional osteogenesis in the maxillary sinus (MS) associated with secondary root canal infection detected using cone-beam computed tomographic (CBCT) imaging. Apical periodontitis, a consequence of root canal infection, may lead to a periosteal reaction in the MS and osteogenesis seen as a radiopaque structure on imaging scans. The use of a map-reading strategy for the longitudinal and sequential slices of CBCT images may contribute to the definition of diagnoses and treatment plans. Root canal infections may lead to reactional osteogenesis in the MS. High-resolution CBCT images may reveal changes that go unnoticed when using conventional imaging. Findings may help define initial diagnoses and therapeutic plans, but only histopathology provides a definitive diagnosis. Surgical enucleation of the periapical lesion is recommended if nonsurgical root canal treatment fails to control apical periodontitis.