Odontogenic infection is a possible cause of maxillary sinusitis. In this case series, the authors evaluated patients who presented with maxillary sinusitis symptoms before and after dental treatment. Intraoral periapical radiographs and cone-beam computed tomography (CBCT) scans were performed on three patients whose maxillary sinusitis symptoms included pain, sinus congestion, respiratory complaints, and what appeared on CBCT to be sinus mucosal thickening. In the first of the three cases, root canal treatment was done. The second case entailed root canal treatment and extraction of the second molar, and in the third case endodontic retreatment was performed. Each patient had a pretreatment CBCT taken and one again at a 4- to 12-month post-treatment follow-up visit. All three patients who presented with moderate-severe sinusitis had reduced symptoms after odontogenic treatment. Post-treatment CBCT confirmed near to complete resolution of sinus mucosal thickening in all patients. These cases show that maxillary sinusitis of odontogenic origin can respond favorably to dental treatment. This case series emphasizes the importance of a medical and dental collaboration when determining treatment for patients who present with maxillary sinusitis. Additionally, the use of CBCT was essential for evaluating the maxillary sinus and its associated odontogenic conditions.