A case of a dentigerous cyst in the mandibular notch associated with actinomycosis is described. The patient was an 87-year-old woman who presented with swelling of the left cheek and trismus. Panoramic radiography and computed tomographic imaging revealed an ectopic impacted tooth with a cystic lesion in the left mandibular notch. A slit-like radiolucent area was observed from the left retromolar region to the cystic lesion. The impacted tooth and cystic lesion were removed with the patient under general anesthesia. The histopathological diagnosis was a dentigerous cyst associated with actinomycetes. After sugery, the patient orally received 500 mg/day of ABPC for 3 weeks. There was no sign of recurrence after treatment.
