Cardiac rehabilitation (CR), a programme of risk factor education and support, is an essential part of the care of people following a heart attack. It reduces cardiovascular-related deaths and hospital readmissions by 25%, yet only about 50% of eligible patients participate in CR. Common communication tools may increase uptake and completion of CR. We aim to assess if existing CR services can be improved by means of a text messaging self-management programme (Text4Heart). A randomised controlled trial, in which 330 participants who are eligible for CR will be randomised to usual CR support plus the Text4Heart intervention for 6 months, or to usual CR alone. Assessments will compare the effect on adherence to taking medication and lifestyle change between the two groups. If effective, this approach could substantially reduce deaths and hospital admissions in a group of patients who account for up to one-third of all hospital admissions.