Aims and objectives: To investigate patients' lived experiences of exercise-based cardiac rehabilitation. Background: Exercise-based cardiac rehabilitation is used to enable patients with cardiac problems to move forward to lead satisfying lives. However, knowledge of patients' concerns while they follow the current programmes is sparse. Design: This study, which included nine men and two women with unstable angina pectoris and non-ST-elevation myocardial infarction, used a phenomenological-hermeneutic approach. Methods: The patients were followed by field observations during exercise-based cardiac rehabilitation. Focus group interviews were conducted at the programme end, and individual interviews were performed one to two months later. The interpretation comprised three methodological steps: naïve reading, structural analysis, and comprehensive interpretation and discussion. Findings: Although both physically and psychologically challenged, the patients were encouraged to maintain an active lifestyle. Three themes were identified: anxiety regarding exercise, whereby the patients are initially insecure about how to behave with their diseased hearts; encouragement from training together, whereby the patients support each other in exercising; and growing confidence in the heart, whereby the patients enjoy being physically active. Conclusions: In exercise-based cardiac rehabilitation, patients' insecurity with respect to their heart disease is revealed as an existential anxiety. Through peer support and a positive physical perception, the patients gain renewed self-efficacy, helping them to continue their lives in an active and satisfying way. Relevance to clinical practice: Knowing that patients are confronted with an existential anxiety during exercise-based cardiac rehabilitation is important because it requires specific care. Recognising this anxiety also highlights how participating in the programme can be very demanding, which can help us understand aspects of adherence problems. Of greatest importance is that exercise-based cardiac rehabilitation enables patients to find a new foothold, which comprises enjoying being physically active.