Recently, the government has moved towards public reporting of 30-day readmission rates after elective primary total knee (TKA) and total hip arthroplasty (THA). We identified 11,814 and 8105 patients who underwent primary TKA and THA from the 2011 ACS NSQIP. Overall readmission rates within 30-days of surgery were 4.6% for TKA and 4.2% for THA. ⋯ In TKA, multivariate analysis identified age (P=0.002), male gender (P=0.03), cancer history (P=0.008), elevated BUN (P=0.002), a bleeding disorder (P<0.001) and high ASA class (P<0.001) as predictors of readmission. In THA, obesity (P=0.008), steroid use (P=0.037), a bleeding disorder (P=0.002), dependent functional status (P=0.022), and high ASA class (P<0.001) predicted readmission. Understanding characteristics associated with readmission will be essential for equitable patient risk stratification.