BACKGROUND: The difficulties of recruiting individuals into mental health trials are well documented. Few studies have collected information from those declining to take part in research, in order to understand the reasons behind this decision. AIM: To explore patients' reasons for declining to be contacted about a study of the effectiveness of cognitive behavioural therapy as a treatment for depression. DESIGN AND SETTING: Questionnaire and telephone interview in general practices in England and Scotland. METHOD: Patients completed a short questionnaire about their reasons for not taking part in research. Semi-structured telephone interviews were conducted with a purposive sample to further explore reasons for declining. RESULTS: Of 4552 patients responding to an initial invitation to participate in research involving a talking therapy, 1642 (36%) declined contact. The most commonly selected reasons for declining were that patients did not want to take part in a research study (n = 951) and/or did not want to have a talking therapy (n = 688) (more than one response was possible). Of the decliners, 451 patients agreed to an interview about why they declined. Telephone interviews were completed with 25 patients. Qualitative analysis of the interview data indicated four main themes regarding reasons for non-participation: previous counselling experiences, negative feelings about the therapeutic encounter, perceived ineligibility, and misunderstandings about the research. CONCLUSION: Collecting information about those who decline to take part in research provides information on the acceptability of the treatment being studied. It can also highlight concerns and misconceptions about the intervention and research, which can be addressed by researchers or recruiting GPs. This may improve recruitment to studies and thus ultimately increase the evidence base.