Little is known about pre- and post-operative weight changes in patients undergoing total joint arthroplasty (TJA). While intra-operative body mass index (BMI) is associated with higher risk of surgical site infections (SSI) and readmission rates post-TJA no studies evaluate the impact of non-surgical pre-operative weight loss and post-operative weight maintenance on risk of these outcomes. This study describes weight changes of patients undergoing TJA, evaluates characteristics associated with weight patterns one year pre- and post-TJA. Additionally, it evaluates whether pre-operative weight loss (5% change) and maintenance post-operatively is associated with SSI and readmission risk post-surgery. A cohort of primary TJAs (total hip arthroplasty (THA) and total knee arthroplasty (TKA)) performed between 01/01/2008 and 12/31/2010 was identified using a Total Joint Replacement Registry (TJRR). Using electronic medical records weight from one year pre- through one year post-TJA patients were categorized into: gainers (increased weight by 5%), losers (decreased by 5%), remained the same (changed of <5%). Patients were characterized by gender, age, race, health status, and co-morbidities. Endpoints, SSI (deep and superficial) and 90 day readmissions, were ascertained prospectively by the TJRR. In 30632 TJAs (10572 THAs and 20060 TKAs) most remained the same weight during the year prior (71.5% THAs and 75.7% TKAs) and year after (64.0% THAs and 68.5% TKAs) the procedure. Women, younger patients, Blacks, and those with certain co-morbidities were more susceptible to weight change pre- and post-TJA. Among obese (BMI<30kg/m_) patients (4066 THAs and 10718 TKAs), few (18.0% THAs and 12.4% TKAs) lost weight prior to TJA or became (6% THA, 3% TKAs) non-obese (BMI <30kg/m_). A significant reduction in risk of SSI and readmission in those who lost 5% of their body weight prior to their TJA procedure was not observed. In obese THA patients who lose weight pre-operatively and keep it off post-operatively (N=444), the likelihood of deep SSI (odds ratio (OR)=3.77, 95% CI 1.59-8.95) is higher than in patients who stayed the same weight. Obese TKA patients who lost weight and kept it off post-operatively (N=937) were at a higher likelihood of readmission (OR=1.63, 95%CI 1.16, 2.28) than those who remained the same weight.