Background: Immediate breast reconstruction with tissue expander in patients who were expected to receive adjuvant therapy, such as chemotherapy or radiotherapy, has been a topic of debate. Postoperative complications from tissue expander procedures can delay the timing of adjuvant treatment and subsequently increase the probability of recurrence. The purpose of this study was to identify the impact of chemotherapy and radiotherapy on postoperative complications in patients who underwent immediate reconstruction (IR) using tissue expander.Method: We conducted a retrospective study of 1,081 breast cancer patients who underwent mastectomy and IR using tissue expander insertion between 2012 and 2017 in Samsung Medical Center. The patients were divided into two groups based on complications (complication group vs. no complication group). The complication group had 59 patients (5.5%) and the no complication group had 1,022 patients (94.5%). Results: In univariate analysis, lymphovascular invasion (LVI), pathologic N stage, pathologic prognostic stage, axillary lymph node dissection (ALND), adjuvant radiotherapy, and adjuvant chemotherapy were significantly associated with postoperative complications. In multivariate analysis, however, higher pathologic N stage was significantly associated with postoperative complications. Chemotherapy or radiotherapy were not risk factors for postoperative complications.Conclusions: IR with tissue expander after mastectomy may be a treatment option even when the patients are expected to receive adjuvant chemotherapy or radiotherapy. These results will aid preoperative counseling to patients who are concerned about the complications of IR caused by chemotherapy or radiotherapy.