To determine the course of adherence to physical activity (PA) recommendation in hip/knee osteoarthritis patients before and after hip/knee arthroplasty (THA/TKA). Moreover, we explored predictors for non-adherence 12 months postoperatively. Primary THA/TKA were included in a multicenter observational study. Preoperatively and 6/12 months postoperatively, patients reported engagement in moderate-intensity PA in days/week in the past 6 months (PA-recommendation (≥30 min of moderate-intensity ≥5 days/week)). We included predictors stratified by preoperative adherence: sex, age, BMI, comorbidities, smoking, living/working status, season, mental health, HOOS/KOOS subscales before and 6 months postoperatively, and 6-month adherence. (1005 THA/972 TKA) Preoperatively, 50% of the population adhered. Adherence increased to 59% at 6 and 12 months. After 12 months, most patients remained at their preoperative PA level, 11% of the preoperative adherers decreased, while 20% of the preoperative non-adherers increased ...