Introduction: People living with cardiac and respiratory disease require improved post-hospital support that is readily available and efficient. Hypothesis/Aims: Support via text message alongside cardiac and pulmonary rehabilitation (rehab) improves clinical, lifestyle and attendance. Aim was to test effectiveness of an automated, semi-personalised text message support program on clinical and lifestyle outcomes among patients attending cardiac and pulmonary rehab and evaluate program acceptability and utility for patients. Methods: Multicentre randomised controlled trial (3:1, intervention:control) among patients attending cardiac and pulmonary rehab. Control received usual care. Intervention received a 6-month text message lifestyle and support program. Primary outcome was 6-minute walk distance (6MWD). Secondary outcomes included clinical measures, lifestyle, medication adherence and rehab program attendance. Results: Of the 316 participants (mean age±standard deviation, 66.7±10.1years) randomised, 60% were male (190/327), 156 were recruited from cardiac rehab. At 6 months, 6MWD improved in both groups and was significantly greater among intervention than control participants (unadjusted mean difference [95% CI]: 43.4m [4.3 to 82.4];p=0.0296). After adjustment for baseline values, there was no significant difference between intervention and control groups for 6MWD (adjusted mean difference 2.2m [-21.2 to 25.6];p=0·85), medication adherence or cardiovascular risk factors. At 6 months, intervention participants reported significantly lower depression scores (adjusted mean diff [95%CI] -1.3 [-2.2 to -0.3], p=0.012) and anxiety (adjusted mean diff [95%CI] -1.1 [-2.1 to 0.0], p=0.046). Sub-group analysis of cardiac rehab participants found a clinically meaningful increase in 6MWD at 6 months in the intervention compared to control group (mean diff 18.9m, 95% CI -29.2 to 67.1). The majority of participants found the messages easy to understand (99%) and useful (86%). Conclusion: An education and support-based text message program for cardiac and pulmonary rehab attendees improved anxiety and depression plus program attendance, was useful for patients and suitable for implementation.