Background With technologic advances such as locked periarticular plating, hemiarthroplasty of the humeral head, and more recently reverse total shoulder replacement, surgical treatment of proximal humerus fractures has become more commonplace. However, there is insufficient information regarding patient outcomes after surgery, such as the frequency of unplanned hospital readmissions and factors contributing to readmission. Questions/purposes We measured (1) the frequency of unplanned hospital readmissions after surgical treatment of proximal humerus fractures, (2) the medical and surgical causes of readmission, and (3) the risk factors associated with unplanned readmissions. Methods The State Inpatient Database from seven different states was used to identify patients who underwent treatment for a proximal humerus fracture with open reduction and internal fixation (ORIF), hemiarthroplasty of the humeral head, or reverse total shoulder arthroplasty from 2005 through 2010. The database was used to measure the 30-day and 90-day readmission rates and identify causes and risk factors for readmission. Multivariate modeling and a Cox proportional hazards model were used for statistical analysis. Results A total of 27,017 patients were included with an overall 90-day readmission rate of 14% (15% for treatment with ORIF, 15% for reverse total shoulder arthroplasty, and 13% for hemiarthroplasty). The majority of readmissions were associated with medical diagnoses (75%), but treatment with ORIF was associated with the most readmissions from surgical complications, (29%) followed by reverse total shoulder arthroplasty (20%) and hemiarthroplasty (16%) (p \ 0.001). Risk of readmission was greater for patients who were female, African American, discharged to a nursing facility, or had Medicaid insurance. Conclusions As the majority of unplanned hospital readmissions were associated with medical diagnoses, it is important to consider patient medical comorbidities before surgical treatment of proximal humerus fractures and during the postoperative care phase. Level of Evidence Level III, therapeutic study. See the Instructions for Authors for a complete description of levels of evidence.