OBJECTIVE: The National Health Service (NHS) in England has introduced a range of policy measures aimed at fostering greater openness, transparency and candour about quality and safety. We draw on the findings of an evaluation of the implementation of these policies in NHS organisations, with the aim of identifying key implications for policy and practice. METHODS: We undertook a mixed-methods policy evaluation, comprising four substudies: a longitudinal analysis of data from surveys of NHS staff and service users; interviews with senior stakeholders in NHS provider organisations and the wider system; a survey of board members of NHS provider organisations and organisational case studies across acute, community and mental health, and ambulance services. RESULTS: Our findings indicate a mixed picture of progress towards improving openness in NHS organisations, influenced by organisational history and memories of past efforts, and complicated by organisational heterogeneity. We identify four features that appear to be necessary conditions for sustained progress in improving openness: (1) authentic integration into organisational mission is crucial in making openness a day-to-day concern; (2) functional and effective administrative systems are vital; (3) these systems must be leavened by flexibility and sensitivity in implementation and (4) a spirit of continuous inquiry, learning and improvement is required to avoid the fallacy that advancing openness can be reduced to a time-limited project. We also identify four persistent challenges in consolidating and sustaining improvement: (1) a reliance on goodwill and discretionary effort; (2) caring for staff, patients and relatives who seek openness; (3) the limits of values-driven approaches on their own and (4) the continued marginality of patients, carers and families. CONCLUSIONS: Variation in policy implementation offers important lessons on how organisations can better deliver openness, transparency and candour. These lessons highlight practical actions for policymakers, managers and senior clinicians.