Attendance rates at cardiac rehabilitation remain low with typically fewer than 35% of eligible patients attending. Much of the poor attendance stems from invited patients failing to attend. To design a theoretically based intervention to improve attendance at cardiac rehabilitation. Our methods followed recommendations that have been developed from the Medical Research Council (MRC) framework for the design of complex interventions. We conducted three processes that progressed simultaneously: 1) literature review for evidence on epidemiology, behavioural theory, and efficacy of interventions; 2) expert meetings on behavioural theory and to select target points for intervention; and 3) development and theoretical modelling of the intervention. Our final interventions were a theoretically worded invitation letter and leaflet based on the Theory of Planned Behaviour and the Common Sense Model of Illness, designed to: a) motivate patients through professional recommendation; b) provide simple information on the contents of cardiac rehabilitation emphasising ease for participants; c) reassure participants that the programme is tailored to their personal needs in a safe supervised environment; and d) reinforce the benefits of attending cardiac rehabilitation. A theoretically worded letter and leaflet could be an inexpensive intervention to improve attendance at cardiac rehabilitation. The letters and leaflets will now be evaluated in a randomised trial.