Professional boundaries make inter-professional communication, collaboration and teamwork more challenging and can jeopardise the provision of safe, high quality patient care. This in-depth interview study conducted in three UK acute hospital organisations in 2003-2004 explored how professional boundaries affected efforts to improve routine practice by acute pain services (small specialist teams set up to drive improvements in postoperative pain management through education, training, standard-setting and audit). The study found that many anaesthetists and to a lesser extent nursing staff saw postoperative pain management as a new and unjustified addition to their professional role. Professional identities and strong fears about the risks of treatments meant that health professionals resisted attempts by the acute pain services to standardise practice and to change medical and nursing roles in relation to postoperative pain management. Efforts by the acute pain services to improve practice were further hindered by inter-professional boundaries (between the medical and nursing professions) and by intra-professional boundaries (within the medical and nursing professions). The inter-professional boundaries led to the acute pain services devoting a substantial part of their time to performing a 'go-between' function between nurses and doctors. The intra-professional boundaries hindered collaborative working among doctors and limited the influence that the acute pain service nurses could have on improving the practice of other nurses. Further work is needed to address the underlying fears that can lead to resistance around role changes and to develop effective strategies to minimise the impact of professional boundaries on patient care.