Background: Postoperative adjuvant therapy is very important in treating breast cancer, and increasing numbers of patients are undergoing immediate breast reconstruction (IBR) at the same time as breast cancer surgery. However, the impact of breast reconstruction surgery on adjuvant therapy is controversial. In this multicenter cohort study led by the Japanese Breast Cancer Society, we examined the impact of reconstructive surgery on postoperative adjuvant therapy in breast cancer patients who underwent IBR. Methods: We examined the association between adjuvant therapy and complications of reconstructive surgery in 4726 patients with primary breast cancer who underwent IBR during the period from January 1, 2008 to December 31, 2016. Statistical analysis employed the χ2 test. Results: Of the 1044 patients who received adjuvant chemotherapy, the median time from the date of surgery to the start of chemotherapy was 51.0 days, with 65 patients (6.2%) starting more than 90 days after surgery. 66 IBR complications required surgery, including infections and tissue expander (TE) removal. Of the 65 patients for whom chemotherapy was initiated 90 days or more after surgery, 9 patients (13.8%) had complications requiring surgery, significantly more than the 57 patients (5.8%) in the < 90 days group (P = 0.010). Post-mastectomy radiation therapy (PMRT) was performed after breast cancer surgery in 359 cases: 69 cases received autologous tissue irradiation, 46 TE irradiation, 241 silicone breast implant (SBI) irradiation, and 3 removal before irradiation. 5 cases given autologous tissue irradiation and 30 receiving artificial material irradiation required surgery within 1 year after the cancer surgery due to complications of IBR. There was no significant difference in the frequency of complications requiring surgery between autologous tissue irradiation and artificial material irradiation (P = 0.422). There were 35 complications (9.7%) requiring surgery in the irradiated group, significantly more than the 118 cases (2.7%) in the non-irradiated group (P < 0.001). Conclusions: Adjuvant chemotherapy was generally not delayed, but complications requiring surgery related to IBR were more frequent in those with delay beyond 90 days, suggesting that postoperative complications from breast reconstruction may have influenced the delay in initiating chemotherapy. Therefore, in cases for whom postoperative chemotherapy is anticipated, complications from breast reconstruction may have impacted outcomes, making preoperative risk assessment even more important. There was no difference in the frequency of reconstructive complication surgery according to whether autologous tissue irradiation or artificial material irradiation had been used, but further investigation is needed.