We report an unusual case of tibial osteomyelitis resulting from Bacille Calmette-Guerin vaccination and causing subsequent cortical destruction and a subcutaneous abscess. A 10-month-old infant initially presented with a tender palpable mass below the left knee. Radiographs showed an osteolytic lesion in the proximal metaphysis of the tibia. Magnetic resonance imaging (MRI) showed a pedicle-based juxtacortical abscess communicating with the cortex and marrow space of the proximal tibia. The mass was excised surgically and tuberculous osteomyelitis was confirmed by polymerase chain reaction. In the present case, MRI demonstrated the mechanism of subcutaneous abscess formation caused by BCG-related osteomyelitis.