A 73-year-old woman who had been followed in our department of gynecology because of ovarian cancer since 2002, was admitted with liver dysfunction and complaining of back pain and light precordial chest pain. The chest radiograph on admission revealed a tumor in her left upper lung field, and chest CT revealed a tumor adjacent to the chest wall and mediastinum. FDG-positron emission tomography (PET) showed abnormal uptake in the tumor and Th6/7, and the subaortic lymph nodes. On the basis of these findings, primary lung cancer with bone metastasis was suspected. She had a high grade fever on admission, and blood cultures were positive for group G streptococcus. The treatment with intravenous penicillin was started. Percutaneous biopsy of the tumor in her left chest showed an abscess wall in the chest wall, but no evidence of malignancy. Transbronchial lung biopsy and CT-guided biopsy also showed no malignant cells. Since the tumor decreased in size and back pain improved gradually by only antibiotic treatment, a diagnosis of sepsis of group G streptococcus, chest wall abscess, and vertebral osteomyelitis was made. She was treated with intravenous penicillin for 4 weeks and oral amoxicillin for another 4 weeks. After 60 days of antibiotic treatment, the tumor vanished.