Inflammatory odontogenic cysts, if not treated, may lead to progression of osteolytic activity, potential paresthesia, and loss of teeth. A 16-year-old female patient was referred by a pediatric dentist for asymptomatic abnormal radiolucency found interproximally to the mandibular left first and second premolars. Radiographic, clinical, and pathologic analyses led to a diagnosis of an inflamed odontogenic cyst (type K09.0) with Actinomyces colonization. The cyst was treated by periodontal regenerative therapy and resulted in successful osseous regeneration. This was a rare case because of the patient's age, the location of the lesion, its association with vital teeth, and its presentation.