This study examined changes in operative mortality, risk of surgical complications, length of stay, and hospital readmission rates before and after mandatory adoption of surgical safety checklists across 101 acute care hospitals in Ontario. None of the surgical outcomes improved after adoption of checklists, in stark contrast to previous observational studies that demonstrated significant improvements in surgical morbidity and mortality following checklist implementation. The risk of surgical complications and mortality prior to checklist implementation was comparable to post-implementation rates in earlier studies; checklists may not be effective when complication rates are below a threshold. In an accompanying editorial, Dr. Lucian Leape suggests that effective checklist use requires adapting the tool to local conditions, training and implementing a new and consistent workflow, and involving the entire health care team. Rather than mandating checklist adoption, he recommends that federal funding support local efforts to implement checklists. In a related piece, Dr. Atul Gawande, who co-authored a major international study that found benefit from surgical checklists and wrote a book about checklists as a safety tool, raises methodological concerns about the Ontario study.