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Background: Conflicting evidence exists with regard to weight loss after total hip arthroplasty or total knee arthroplasty. The purposes of this study were to determine whether patients lose weight after total hip arthroplasty or total knee arthroplasty, whether there are predictors of weight change after total hip arthroplasty or total knee arthroplasty, and whether weight changes after total hip arthroplasty or total knee arthroplasty affect patient-reported outcomes., Methods: Using our institutional registry, we evaluated the two-year change in self-reported body mass indices for all patients who underwent elective, unilateral total hip arthroplasty or total knee arthroplasty for osteoarthritis. A 5% change in body mass index was considered clinically meaningful. Patient-reported outcomes were compared between patients who underwent total hip arthroplasty and those who underwent total knee arthroplasty and between obesity classes, on the basis of whether patients gained, lost, or maintained weight., Results: We reviewed 3893 total hip arthroplasties and 3036 total knee arthroplasties. Of the patients who underwent total joint arthroplasty, 73% (2850 patients) in the total hip arthroplasty group and 69% (2090 patients) in the total knee arthroplasty group demonstrated no change in body mass index. Patients who underwent total knee arthroplasty were more likely to lose weight than patients who underwent total hip arthroplasty. Increasing preoperative obesity correlated with a greater likelihood of weight loss. Patients who underwent total knee arthroplasty and lost weight demonstrated better clinical outcome scores, but weight gain in general was associated with inferior clinical outcomes. Greater body mass index, total knee arthroplasty, and female sex were significant predictors of weight loss (p < 0.05). Better preoperative functional status was significantly associated with a lower likelihood of weight gain (p < 0.05)., Conclusions: Most patients maintained their body mass index after total hip arthroplasty or total knee arthroplasty. Female patients, patients with higher preoperative body mass index, and those who underwent total knee arthroplasty were more likely to lose weight after surgery., Level of Evidence: Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence., (Copyright © 2015 by The Journal of Bone and Joint Surgery, Incorporated.)