To assess the burden of post-discharge healthcare utilization given by readmissions beyond 30-days following immediate breast reconstruction (IBR) nationwide. Women with breast cancer who underwent mastectomy and concurrent IBR (autologous and implant-based) were identified within the 2010-2019 Nationwide Readmission Database. Cox proportional hazards and generalized linear regression controlling for patient- and hospital-level confounders were used to determine factors associated with 180-day unplanned readmissions and incremental hospital costs, respectively. Within 180 days 10.7% of 100,942 women were readmitted following IBR.. Readmissions tended to be publicly insured (30.8% vs. 21.7%, P<0.001), and multimorbid (Elixhauser comorbidity index >2 31.6% vs. 19.6%, P<0.001) compared to non-readmitted patients. There were no differences in readmission rates amongst types of IBR (tissue expander 11.2%, implant 10.7%, autologous 10.8%; P>0.69). Of all readmissions, 40% occurred within 30 days and 21.7% in a different hospital, and 40% required a major procedure in the operating room. Infection was the leading cause of readmissions (29.8%). In risk-adjusted analyses, patients with carcinoma in situ, publicly insured, low socioeconomic status, and higher comorbidity burden were associated with increased readmissions (all P<0.05). Readmissions resulted in additional $8,971.78 (95% CI: $8,537.72-9,405.84, P<0.001) in hospital costs which accounted for 15% of the total cost of immediate breast reconstruction nationwide. The majority of inpatient healthcare utilization given by readmissions following mastectomy and IBR occurs beyond the 30-day benchmark. There is evidence of fragmentation of care as a quarter of readmissions occur in a different hospital and over one-third require major procedures. Mitigating postoperative infectious complications could result in the highest reduction of readmissions.