We report the case of a 38-year old male patient with sinusitis caused by an infected follicular cyst due to an ectopic impacted third molar in the right maxillary sinus. A 10-day antibiotherapy regimen was administered; subsequently, the cyst and the third molar were removed achieving complete recovery. Fibrous dysplasia was diagnosed at follow-up examination (occupation of the maxillary sinus by bone tissue was observed in a radiographic examination) and confirmed by biopsy. In cases of odontogenic sinusitis, thorough examination is crucial, as evidenced by the case reported in this study. A Literature review was performed in order to identify the diagnostic methods currently available and the clinical features, complications and treatment for both, odontogenic maxillary sinusitis and fibrous dysplasia.