A case of idiopathic external root resorption identified in a girl aged 4 years and 2 months (4Y2M) is presented. The patient visited our clinic with a chief complaint of spontaneous pain in the area of the lower left primary canine. An intraoral examination revealed severe mobility of the affected tooth, while its marginal gingiva was swollen with a maximum periodontal pocket depth of 5 mm. Irrigation of the periodontal pocket and general administration of antibiotics relieved the symptoms

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... in 1 week. However, spontaneous pain recurred in the same area at 5Y0M. External root resorption was found with alveolar bone loss on the distal side, and microbiological analyses using broadrange polymerase chain reaction and sequencing methods were carried out with saliva and subgingival dental plaque specimens. In dental plaque specimens from unaffected regions and saliva specimens, Neisseria and Streptococcus species were frequently identified throughout the observation period. On the other hand, Prevotella and Fusobacterium species were predominant species in the dental plaque specimens from the area of the affected tooth. These results indicate that periodontopathic bacterial species rarely seen in children may have inhabited periodontal pockets formed from external root resorption in the present case.