Patient involvement in decision-making is widely regarded as an important feature of good-quality healthcare. Policy-makers have been particularly concerned to ensure that patients are informed about and enabled to choose between relevant treatment options, but it is not clear how patients understand and value involvement. We investigated the meaning of involvement in treatment decision-making for people with diabetes. We conducted semi-structured interviews with 18 people aged between 20 and 79 who had type 1 or type 2 diabetes selected from 4 multi-practitioner outpatient clinics in the Grampian area of Scotland. We used several strategies to probe their understandings of involvement, including a discussion of how they would respond to a question about involvement in treatment decisions that appears on the National Patient Survey used to monitor the quality of healthcare in England. Participants associated involvement in decision-making with a number of features relating to the ethos and feel of healthcare encounters (welcoming; respectful; facilitative of patients' contributions; and non-judgmental); communication about health problems (practitioners attending to patients' views and patients feeling listened to; practitioners giving clear explanations based on their professional knowledge and patients understanding these); and communication about treatments (practitioners explaining treatment rationales in ways that patients understand and enabling patients to feel they have a say). Our findings have implications for practical attempts to involve patients in decisions about their care and for the conceptualisation and assessment of patient involvement. They suggest that practitioners who aspire to facilitate patient involvement should attend to the ethos they foster in consultations and the way they discuss problems as well as to the provision of information about treatment options and the scope patients have to influence decisions. Models and taxonomies of patient involvement in decision-making need to be developed to accommodate both problem-solving phases and the relational and subjective dimensions of involvement.