Background: The optimal treatment strategy following primary total hip or knee replacement remains unknown. The purpose of this study was to evaluate the effect of ergometer cycling after hip or knee replacement surgery on health-related quality of life and patient satisfaction. Methods: Three hundred and sixty-two patients were randomly assigned to either perform or not perform ergometer cycling beginning two weeks after total hip or knee replacement. The primary outcome was self-reported physical function as measured with the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) at three, six, twelve, and twenty-four months postoperatively. Results were compared with published thresholds for minimal clinically important improvements. Results: The baseline characteristics of the two groups were similar. After the hip arthroplasties, all of the outcome parameters were superior in the ergometer cycling group at all follow-up intervals, and the primary outcome, physical function as measured with the WOMAC, was significantly better at three months (21.6 compared with 16.4 points, effect size = 0.33, p = 0.046) and twenty-four months (14.7 compared with 9.0 points, effect size = 0.37, p = 0.019). After the hip arthroplasties, a higher percentage of the ergometer cycling group was ''very satisfied'' at all follow-up intervals (for example, 92% compared with 80% at three months; p = 0.027). The significant differences in the primary outcome exceeded the absolute minimal clinically important improvement threshold by a factor of 2.0. No significant differences between the study groups were seen after the knee arthroplasties. Conclusions: Ergometer cycling after total hip arthroplasty is an effective means of achieving significant and clinically important improvement in patients' early and late health-related quality of life and satisfaction. However, this study does not support the use of ergometer cycling after knee arthroplasty. Level of Evidence: Therapeutic Level I. See Instructions to Authors for a complete description of levels of evidence.