Roentgenography such as a sinus routine series is a simple and inexpensive method widely used in screening paranasal sinus diseases. However, the sensitivity and specificity of a plain film to confirm the etiology and severity of paranasal sinus diseases are not satisfactory. In this paper, we present a case of odontogenic cyst accompanied with ipsilateral maxillary sinusitis. Our case was a 36-year-old man who received a transnasal, endoscopic sinus surgery four years ago. However, a cyst at the maxillary floor was overlooked during that procedure and maxilla deformity ensued. This reminds us that only using a plain film is not sufficient enough to diagnose sinus diseases. We recommend that computed tomography should be performed when conventional radiography does not provide sufficient information. Furthermore, we would like to emphasize that an odontogenic disease may be an easily unrecognized cause of a maxillary sinus lesion.