A 62-year-old woman presented with a right breast tumor. Examination yielded a diagnosis of right breast cancer (T4bN3bM1[lung]). After right breast mastectomy for local control, 9 sessions of chemotherapy containing an anthracycline and taxane were administered. Because of progression of lung metastasis, bevacizumab and paclitaxel were administered. Follow-up computed tomography(CT)scans showed that the multiple solid lung metastases had reduced in size, but a hollowing effect was noted. Eight months later, the patient developed left pneumothorax. The origin of the pneumothorax was unclear, but the hollowing due to the lung metastases seen after bevacizumab treatment was thought to be the cause. The pneumothorax resolved after tube thoracostomy, but disease progression occurred because of the lack of chemotherapy, and the patient's general condition worsened. Although rare, we speculate that there is a risk of pneumothorax after bevacizumab- containing chemotherapy.