Background: New medical technologies are often used widely without adequate supporting data, a practice that can lead to widespread catastrophic failure such as occurred with metal-on-metal (MoM) hip replacements. We determined both how revision rates would have differed if, instead of receiving MoM hip replacements, patients had received existing alternatives and the subsequent cumulative re-revision rates of the patients who did receive MoM hip replacements compared with alternatives. Methods: This study is a population-based longitudinal cohort study of patient data recorded in the National Joint Registry (NJR) for England, Wales and Northern Ireland between April 2003 and December 2014. We ascertained implant failure rates separately among stemmed MoM total hip replacement (THR) and hip-resurfacing procedures and, using flexible parametric survival modeling, compared them with the failure rates that would have been expected had existing alternatives been used. We used Kaplan-Meier survivorship analysis to compare cumulative re-revision of patients who received stemmed MoM primary replacements that failed and of those who underwent hip resurfacing that failed with those whose non-MoM THRs had failed. Results: In all, 37,555 patients underwent MoM hip resurfacing, with a 10-year revision rate of 12.6% (95% confidence interval [CI]: 12.2% to 13.1%) compared with a predicted revision rate of 4.8% if alternative implants had been used. The 32,024 stemmed MoM THRs had a 19.8% (95% CI: 18.9% to 20.8%) 10-year failure rate compared with an expected rate of 3.9% if alternatives had been used. For every 100 MoM hip-resurfacing procedures, there were 7.8 excess revisions by 10 years, and for every 100 stemmed MoM THR procedures, there were 15.9, which equates to 8,021 excess first revisions. Seven-year re-revision rates were 14.9% (95% CI: 13.8% to 16.2%) for stemmed non-MoM THRs, 18.0% (95% CI: 15.7% to 20.7%) for MoM hip resurfacing, and 19.8% (95% CI: 17.0% to 23.0%) for stemmed MoM THRs. Conclusions: This study highlights the consequences of widespread and poorly monitored adoption of a medical technology. Over 1 million MoM hip prostheses were implanted worldwide. The excess failure on a global scale will be enormous. This practice of adopting new technologies without adequate supporting data must not be repeated. Level of Evidence: Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.