Objectives: To ascertain the acceptability of a randomised controlled trial comparing arthroscopic lavage with a placebo-surgical procedure for the management of osteoarthritis of the knee; and to assess the practical feasibility of mounting such a multicentre placebo-controlled trial. Design: Mixed methods study including: focus groups with surgeons and anaesthetists; focus groups and interviews with potential participants; interviews with chairpersons of UK Multicentre Research Ethics Committees (MRECs); surveys of surgeons and anaesthetists; and a two-centre, three-arm pilot. Setting: UK secondary care. Participants: Members of the British Association of Surgeons of the Knee and members of the British Society of Orthopaedic Anaesthetists took part in the focus groups and surveys. Surgeons and anaesthetists from two regional centres in the UK also contributed to focus groups, as did patients from consultant lists in two UK regional centres, and members of Arthritis Care. Chairpersons of six UK MRECs were interviewed. Participants were eligible for the pilot if they were adults (18 years or older) with radiological evidence of osteoarthritis of the knee who might be considered for arthroscopic lavage, and were fit for general anaesthetic (defined by the American Society of Anaesthesiologists grades 1 and 2), and able to give informed consent. Interventions: Participants in the pilot study were randomised to arthrosocopic lavage (with or without debridement at the clinical discretion of the surgeon); placebo surgery; or non-operative management with specialist reassessment. Main outcome measures: The acceptability and feasibility of mounting a placebo-controlled trial for the evaluation of knee arthroscopic lavage. Main outcome measures: The acceptability and feasibility of mounting a placebo-controlled trial for the evaluation of knee arthroscopic lavage. Results: There was broad acceptance across all stakeholder groups of the need to find out more about the effectiveness of arthroscopic lavage. Despite this there was variation in opinion within all the groups about how researchers should approach this and whether or not it would be acceptable to investigate using placebo surgery. Within the health professional groups, there tended to be a split between those who were strongly opposed to the inclusion of a placebo surgery arm and those who were more in favour. For prospective trial participants who had osteoarthritis of the knee, the acceptability of the trial was discussed from a more individual perspective – reflecting on their personal reasons for or against participating. The majority of this group said they would consider taking part. The pilot study showed that, in principle, a placebo-controlled trial could be conducted. It showed that patients were willing to participate in a trial which would involve a placebo-surgical arm and that it was possible to undertake placebo surgery successfully and to blind patients to their allocation – although once patients knew their allocation, some patients allocated to surgery became more concerned about the possibility of undergoing placebo surgery, and withdrew. The experience of the pilot, however, showed that, despite full MREC approval, the study required major discussion and negotiation before local clinical approvals could be obtained. The fact that ethics approval had been granted did not mean that clinicians would automatically accept that the process was ethical. Conclusions: The study showed that, in principle, a placebo-controlled trial of arthroscopic lavage could be conducted in the UK, albeit with difficulty. Against the background of falling use of arthroscopic lavage the decision was, therefore, taken not to proceed to full-scale trial for this procedure. The study showed that for some health professionals the use of placebo surgery can never be justified. It highlighted the importance of the surgeon–anaesthetist relationship in this context and how acceptance of the trial design by both parties is essential to successful participation. It also highlighted the importance of informed consent for trial participants and the strength and influence of individuals' ethical perspectives in addition to collective ethics provided by MRECs.