AbstractA 4‐year‐old Japanese boy was referred to Osaka University Dental Hospital because of severe mobility and pain of the right lower primary canine. The canine had severe bone loss and a pocket depth exceeding 5‐6 mm. The left lower canine showed slight mobility and moderate alveolar bone loss. The other primary teeth showed no pathogenic findings. The subgingival microflora from the right lower canine was dominated by gram‐negative rods, especially capnocytophaga and fusobacterium, while actinomyces sp. were the most common gram‐positive bacteria. While neutrophil functions of the patient were within the normal ranges of healthy subjects, some lymphocyte functions such as IL‐2 production and IgG and IgM syntheses were lower in the patient. 7 months after the extraction of the right lower primary canine, the patient complained of pain around the right lower primary lateral incisor. In 3‐4 weeks, the alveolar bone was lost rapidly and mobility of the lower anterior teeth increased significantly. The primary lateral incisor was extracted and the other primary teeth were treated by scaling and systemic and local administration of antibiotics. After treatment, the lower anterior teeth became less mobile and the gram‐positive coc