A 43-year-old woman presented to our hospital with a right breast tumor. Core needle biopsy revealed invasive ductal carcinoma, which was estrogen and progesterone receptor-negative, HER2-negative, and Ki-67-positive (80%); she was diagnosed with stage IIA right-sided breast cancer. She was asymptomatic with no fever and had no significant medical or family history. Laboratory tests showed inflammation (white blood cells [WBC]: 10,800/μL and C-reactive protein [CRP]: 6.48 mg/dL). There were no other abnormal findings, or derangements in liver and kidney function. Based on the profile of triple-negative breast cancer, dose-dense epirubicin–cyclophosphamide (EC) (epirubicin 90 mg/m2 day 1 + cyclophosphamide 600 mg/m2 day 1 bi-weekly × 4 cycles) followed by paclitaxel (175 mg/m2 on day 1 bi-weekly × 4 cycles) was planned as preoperative chemotherapy. PEG-G was administered 3 days after the first EC treatment. On day 8, the patient developed a fever of 38.3 °C, and was admitted on day 11 due to lingering fever. On admission, her body temperature was 39.4 °C; the laboratory data showed: leukocytosis (WBC: 28,700/µL), elevation of CRP (27.1 mg/dL), prothrombin time/activated partial thromboplastin time (PT/APTT) prolongation (14.6%/46.8 s), elevation of D-dimer level (2.83 µg/mL), and liver dysfunction (aspartate aminotransferase, 356 U/L; alanine aminotransferase, 536 U/L). Immunological tests revealed a 40-fold lower level of anti-nuclear antibodies; myeloperoxidase anti-neutrophil cytoplasmic antibody and proteinase 3 anti-neutrophil cytoplasmic antibody tests were negative. Her immunoglobulin G4 level was normal, and she tested negative for mumps virus, mycobacterium tuberculosis, primary biliary cirrhosis, Epstein–Barr virus, and cytomegalovirus infection. Urinalysis showed no abnormal findings. The blood and urine cultures were negative. A CT scan revealed diffused wall thickening centered on the aortic arch, suggesting vasculitis. Carotid echocardiography showed no clear signs of inflammation. Although the bacterial cultures were negative, she was treated with antibiotics (tazobactam/piperacillin 4.5 g, four times a day) commencing on day 11. However, they were discontinued on day 18 due to the deterioration of her general condition. PEG-G-induced aortitis was suspected based on the CT scan and the ineffectiveness of antibiotics. She was then treated with 60 mg of high-dose prednisolone (1.0 mg/kg/day), which led to a rapid improvement in her general condition and laboratory findings. The CRP levels were within the normal range (< 0.30 mg/dL) on day 36, and the dose of prednisolone was reduced to 45 mg/day. On day 39, the wall thickening of the aortic arch decreased, and she was discharged on day 43. Two months after steroid treatment, the patient underwent breast-conserving surgery with sentinel lymph node biopsy. Her postoperative treatment consisted of chemotherapy followed by irradiation (42.5 Gy/16 Fr; performed 3 months after steroid treatment). Chemotherapy with EC can potentially cause hepatic and renal disorders, and taxane anticancer drugs such as docetaxel and paclitaxel, which are the standard treatment, may cause myelosuppression requiring G-CSF use. Additionally, postoperative oral capecitabine has been reported to improve prognosis []. Therefore, capecitabine was started 4 months after steroid treatment (at a dose of 1000 mg/m2 twice a day for days 1–14 of a 21-day cycle; 8 courses were planned in total). The patient is currently undergoing follow-up with no treatment, and the breast cancer has not recurred. During steroid treatment, prednisolone was administered orally with doses gradually decreasing from 30 mg after discharge; 15, 12, and 10 mg were administered during surgery, irradiation, and anticancer drug therapy, respectively. Prednisolone is being continued 1 year after the onset, at a dose of 5 mg, and there has been no relapse of vasculitis.