Background: There has been a major and alarming increase in readmission rates following total joint arthroplasty. With proposed changes in reimbursement policy, increased rates of unplanned readmission following arthroplasty will penalize providers. In particular, it has been proposed that specific complications-so-called ''zero-tolerance'' complications-are unacceptable and that their treatment will not qualify for reimbursement. The purpose of this study was to identify the incidence, causes, and risk factors for readmission following total joint arthroplasty. Methods: An institutional arthroplasty database was utilized to identify those patients undergoing total knee or hip arthroplasty from January 2004 through December 2008. A total of 10,633 admissions for primary arthroplasty (5207 knees and 5426 hips) were identified. The same database was used to identify patients requiring an unplanned readmission within ninety days of discharge. Multivariate logistic regression was utilized to determine the independent predictors of readmission within ninety days. Results: There were 591 unplanned readmissions within ninety days of discharge following 564 (5.3%) of the 10,633 admissions for total joint arthroplasty. The most common cause of readmission was joint-related infection, followed by stiffness. Black race, male sex, discharge to inpatient rehabilitation, increased duration of hospital stay, unilateral replacement, decreased age, decreased distance between home and the hospital, and total knee replacement were independent predictors of readmission within ninety days. Conclusions: The high incidence of readmissions secondary to potential ''zero-tolerance'' events suggests that these are not easily preventable complications. In addition, longer hospitalization and discharge to an inpatient continued-care facility increased the risk of readmission. Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.