Study objectives-To understand reasons for compliance and non-compliance with a home based exercise regimen by patients with osteoarthritis of the knee. Design-A qualitative study, nested within a randomised controlled trial, examining the eVectiveness of physiotherapy in reducing pain and increasing mobility in knee osteoarthritis. In the intervention arm, participants undertook a series of simple exercises and repositioning of the kneecap using tape. In depth interviews were conducted with a subset of participants in the intervention arm using open ended questions, guided by a topic schedule, to encourage patients to describe their experiences and reflect on why they did or did not comply with the physiotherapy. Interviews were audiotaped, fully transcribed and analysed thematically according to the method of constant comparison. A model explaining factors influencing compliance was developed. Setting-Patients were interviewed at home. The study was nested within a pragmatic randomised controlled trial. Participants-Twenty participants in the intervention arm of the randomised trial were interviewed three months after they had completed the physiotherapy programme. Eight were interviewed again one year later. Main results-Initial compliance was high because of loyalty to the physiotherapist. Reasoning underpinning continued compliance was more complex, involving willingness and ability to accommodate exercises within everyday life, the perceived severity of symptoms, attitudes towards arthritis and comorbidity and previous experiences of osteoarthritis. A necessary precondition for continued compliance was the perception that the physiotherapy was eVective in ameliorating unpleasant symptoms. Conclusions-Non-compliance with physiotherapy, as with drug therapies, is common. From the patient's perspective, decisions about whether or not to comply are rational but often cannot be predicted by therapists or researchers. Ultimately, this study suggests that health professionals need to understand reasons for noncompliance if they are to provide supportive care and trialists should include qualitative research within trials whenever levels of compliance may have an impact on the eVectiveness of the intervention. (J Epidemiol Community Health 2001;55:132-138) Research has consistently shown that up to half of all patients do not follow recommended drug regimens.