Various pieces of information, including the age of the patient, the site involved, symptoms, intraoral findings, and X-ray findings are necessary in making a clinical diagnosis. This report describes a case in which the initial diagnosis was eruption sequestrum based on clinical information but which was later found to be a complex odontoma based on histopathological findings. The patient was an 8-year-old girl who presented at our hospital with the chief complaint of pain in the right mandibular molar region. The distal aspect of the right mandibular first molar, which had partially erupted, was covered with gingiva and hard tissue which were separated from the tooth itself. X-ray findings showing radiopaque hard tissue at this site indicated a clinical diagnosis of eruption sequestrum. After local infiltration anesthesia, the hard tissue was extracted and a gingivectomy performed. Histopathological examination of the excised material, however, indicated a diagnosis of complex odontoma.