Objectives: We sought to clarify how large a proportion of fatal medical accidents can be considered to be caused by poor Non-Technical Skills, and to support development of a policy to reduce numbers of such accidents by making recommendations about possible training requirements. Design: Summaries of reports of fatal medical accidents, published by the Japan Medical Safety Research Organization, were reviewed individually. Three experienced clinicians and one patient safety expert conducted the reviews to determine the cause of death. Views of the patient safety expert were given additional weight in the overall determination. Setting: A total of 73 summary reports of fatal medical accidents were reviewed. These reports had been submitted by healthcare organisations across Japan to the Japan Medical Safety Research Organization between April 2010 and March 2013. Primary and secondary outcome measures: The cause of death in fatal medical accidents, categorised into technical skills, non-technical skills, and inevitable progress of disease were evaluated. Non-technical skills were further sub-divided into situation awareness, decision-making, communication, team working, leadership, managing stress, and coping with fatigue. Results: Overall, the cause of death was identified as non-technical skills in 34 cases (46.6%), disease progression in 33 cases (45.2%), and technical skills in two cases (5.5%). In two cases, no consensual determination could be achieved. Further categorisation of cases of non-technical skills were identified 14 cases (41.2%) of problems with situation awareness, eight (23.5%) with team-working, and three (8.8%) with decision-making. These three sub-categories, or combinations of them, were identified as the cause of death in 33 cases (97.1%). Conclusions: Poor non-technical skills were considered to be a significant cause of adverse events in nearly half of the fatal medical accidents examined. Improving non-technical skills may be effective for reducing accidents, and training in particular sub-categories of non-technical skills may be especially relevant.