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OBJECTIVES To use the Theoretical Domains Framework (TDF) to identify barriers and enablers to participant retention in trials requiring questionnaire return and/or attendance at follow-up clinics. STUDY DESIGN AND SETTING We invited participants (n=607) from five pragmatic effectiveness trials, who missed at least one follow-up time point (by not returning a questionnaire and/or not attending a clinic visit), to take part in semi-structured telephone interviews. The TDF informed both data collection and analysis. To establish what barriers and enablers most likely influence the target behaviour the domain relevance threshold was set at >75% of participants mentioning the domain. RESULTS Sixteen participants (out of 25 showing interest) were interviewed. Overall, seven theoretical domains were identified as both barriers and enablers to the target behaviours of attending clinic appointments and returning postal questionnaires. Barriers frequently reported in relation to both target behaviours stemmed from participants' knowledge, beliefs about their capabilities and the consequences of performing (or not performing) the behaviour.. Two domains were identified as salient for questionnaire return only: goals; and memory, attention and decision-making. Emotion was identified as relevant for clinic attendance only. CONCLUSION This is the first study informed by behavioural science to explore trial participants' accounts of trial retention. Findings will serve as a guiding framework when designing trials to limit barriers and enhance enablers of retention within clinical trials.