OBJECTIVES: The number of primary total hip arthroplasties (THAs) and total knee arthroplasties (TKAs) continues to increase steeply in the US. It is unknown if this trend is the same in Germany and the United Kingdom (UK) as these populations age and become heavier. The purpose of our study was to investigate recent trends of primary THA and TKA in Germany and the UK. METHODS: We analyzed data from the German Bundesgeschäftsstelle für Qualitätssicherung and the UK’s National Joint Register. RESULTS:   Between 2004 and 2008 in Germany, THAs increased 14% and TKAs increased 32%. During the same time in the UK, there were 60% and 70% increases in THAs and TKAs, respectively. In the UK, more TKAs than THAs have been performed since 2005, and a similar trend is expected in Germany after 2010. Nearly 20% of TKAs in Germany were performed on non-elderly patients (64 or younger) and patient age distributions remained approximately the same over the study period. In the UK, however, approximately 30% of TKAs were on non-elderly patients and the proportion of non-elderly patients that underwent TKA has increased every year. CONCLUSIONS: The number of arthroplasties in the US has increased dramatically among non-elderly adults. We observed no increase in this patient group in Germany but a slow and gradual increase in the UK. As obesity has risen over the last decades, the US has experienced greater inclines in TKAs than THAs; the annual number of TKAs is now twice that of THAs. Although the number of TKAs and the number of THAs had been approximately the same in Germany and the UK, recent trends suggest a growing gap due to a steeper rise in TKAs.  Obesity, a strong risk factor for TKA, may be a bigger contributor than aging to the recent growth in joint arthroplasty.