“ Off-label use” refers to medical device utilization for purposes or subpopulations other than those approved by the United States Food and Drug Administration. The primary goal of this study was to determine the current epidemiology of off-label total hip and knee arthroplasty (THA and TKA, respectively) in the United States and to project further off-label use through 2040. Over the past decade, the prevalence of off-label THA and TKA was 30.4% and 37.0%, respectively, growing ~70% from 2000 2010. By 2040, the majority of THA (86.1%) and TKA (91.5%) could be off-label. The high prevalence of off-label arthroplasty and the dramatically shifting patient profile illustrated by these results highlight the need for continued medical device surveillance among on-and off label patients.