A case of infantile malignant osteopetrosis with refractory mandibular osteomyelitis is reported. A female child was diagnosed with osteopetrosis at 2 years 8 months of age, and was scheduled to receive a bone marrow transplantation (BMT) in a pediatric department at 3 years 11 months of age. Her lower incisors were extracted at a dental clinic, after which she had recurring abscesses with progressive severity. She was referred to our department to control local infections before the BMT and was diagnosed with chronic mandibular osteomyelitis caused by osteopetrosis. Sequestrotomy and curettage were performed under general anesthesia. After surgery, daily saline irrigation was continued and osteomyelitis was well controlled. An uneventful BMT was performed, although it was refused and failed. A second BMT was planned, but during chemotherapy, cellulitis occurred after a recurrence of osteomyelitis caused by a newly erupted tooth. The local infection was controlled, but pneumonia recurred. She ultimately died of respiratory failure at 5 years of age.