Introduction: Physical activity is a major outcome in total hip arthroplasty (THA) and discharge criterion. Increasing immediate post-op activity may accelerate discharge, enable fast track surgery and improve general rehabilitation. Preliminary evidence (O'Halloran P.D. et al. 2015) shows that feedback via motivational interviewing can result in clinically meaningful improvements of physical activity. It was the aim of this study to use wearable sensor activity monitors to provide and study the effect of biofeedback on THA patients' activity levels. It was hypothesized that biofeedback would increase in-hospital and post-discharge activity versus controls. Methods: In this pilot study, 18 patients with osteoarthritis receiving elective primary THA followed by a rapid recovery protocol with discharge on day 3 after surgery (day 0) were randomized to the feedback group (n=9, M/F: 4:5, age 63.3 ± 5.9 years, BMI 26.9 ± 5.1) or a non-feedback control group (n=9, M/F: 0:9, age 66.9 ± 5.1 years, BMI 27.1 ± 4.0). Physical activity was measured using a wearable sensor and parameters (Time-on-Feet (ToF), steps, sit-stand-transfers (SST), mean cadence (steps/min)) were calculated using a previously validated algorithms (Matlab). For the in-hospital period data was calculated twice daily (am, ca. 8–13:00h and pm, ca. 13–20:00h) of day 1 (D1) and 2 (D2). The feedback group had parameters reported back twice (morning, lunch) using bar charts comparing visually and numerically their values (without motivational instructions) to a previously measured reference group (n=40, age 71 ±7 years, M:F 16:24) of a conventional discharge protocol (day 4/5). Activity measures continued from discharge (D3) until day 5 (D5) at home. Results: Randomization resulted in matched groups regarding age and BMI, but not gender. The first post-op activity assessment (D1am) was identical between groups. Also thereafter similar values with no significant differences in any parameter were seen, e.g. the time-on-feet at D2PM was 59.2 ±31.7min (feedback) versus 62.9 ±39.2min (controls). Also on the day of discharge and beyond, no effect from the in-hospital feedback was measured. For both groups the course of activity recovery showed a distinct drop on day 4 following a highly active day of discharge (D3). On day 5, activity levels only recovered partially. For both groups, all quantitative activity parameters were significantly higher than the reference values used for feedback. Only cadence as a qualitative measure was the same like reference values. Discussion: Biofeedback using activity values from a body-worn monitor did not increase in-hospital or immediate post-op home activity levels compared to a control group when using the investigated feedback protocol. In general, while the day of discharge steeply boosts patient activity, the day after at home results in an activity drop to near in-patient levels before discharge. In a fast track surgery protocol, it may be of value to avoid this drop via patient education or home physiotherapy. Biofeedback using activity monitors to increase immediate post-op activity for fast track surgery or improved recovery may only be effective when feedback goals are set higher, are personalised or have additional motivational context