A 67-year-old woman, receiving oral steroid therapy for SLE, presented to her physician because of fever (>38  C) and arthralgia. Her steroid dosage was increased with suspicion of SLE exacerbation. Subsequently, she developed redness, swelling, and blistering on the right thumb, and then was referred to our hospital. Clinical examination revealed erythematous swelling on the right thumb with erosion and maceration (Fig. 1A). We diagnosed it as bacterial cellulitis and treated her with antibiotics and antiviral agents, but her symptoms did not improve. Therefore, we performed skin biopsy of the right thumb. Histopathologic examination revealed caseous necrosis surrounded by histiocytes and a positive result was obtained for acid-fast bacilli by Ziehl-Neelsen staining (Fig. 1B-D). In addition, polymerase chain reaction (PCR) assay of the biopsy specimen identified tubercle group bacilli. Based on these results, we made a diagnosis of TB cellulitis and started to treat the patient with isoniazid, ethambutol, rifampicin, and pyrazinamide. Her symptoms completely resolved after the treatment.