In this article we report a case of a 54-year-old man with a primordial cyst in the left maxillary molar region. We performed conventional radiographs and computed tomography (CT) without contrast enhancement, but it was difficult to determine whether the lesion was a tumor or a cyst. CT images with contrast medium showed a cyst, not a tumor. Histopathologically, the cyst wall was composed of fibrous connective tissue lined by nonkeratinized squamous epithelium. Based on its clinical, radiological, and histopathological findings, this lesion was diagnosed as a primordial cyst. This case indicated that enhanced CT imaging is useful for differentiating between cysts and benign tumors and that there are no specific differences in the radiological appearance between primordial cysts and odontogenic keratocysts © 2006 Japanese Society for Oral and Maxillofacial Radiology Springer-Verlag.