QUALITY ISSUE: Approximately 10% of patients are harmed by healthcare, and of this harm 15% is thought to be medication related. Despite this, medication safety data used for improvement purposes are not often routinely collected by healthcare organizations over time. INITIAL ASSESSMENT: A need for a prospective medication safety measurement tool was identified. CHOICE OF SOLUTION: The aim was to develop a tool to allow measurement and aid improvement of medication safety over time. The methodology used for the National Health Service (NHS) Safety Thermometer was identified as an approach. The resulting tool was named the 'Medication Safety Thermometer'. IMPLEMENTATION: The development of the Medication Safety Thermometer was facilitated by a multidisciplinary steering group using a Plan, Do, Study, Act (PDSA) method. Alpha and beta testing occurred over a period of 9 months. The tool was officially launched in October 2013 and continued to be improved until May 2016 using ongoing user feedback. EVALUATION: Feedback was gained through paper and online forms, and was discussed at regular steering group meetings. This resulted in 16 versions of the tool. The tool is now used nationally, with over 230 000 patients surveyed in over 100 NHS organizations. Data from these organizations are openly accessible on a dedicated website. LESSONS LEARNED: Measuring harm from medication errors is complex and requires steps to measure individual errors, triggers of harm and actual harm. PDSA methodology can be effectively used to develop measurement systems. Measurement at the point of care is beneficial and a multidisciplinary approach is vital.