Objective: To describe the experience of physical activity in patients with heart failure who were randomized to a control group in which they received exercise advice and motivational support. Design: A qualitative descriptive interview study, using qualitative content analysis. Patients: Fifteen patients with heart failure. Methods: Interviews with members of the control group in a physical activity trial (Heart Failure-Wii study), who received exercise advice and telephone follow-ups. Results: The experience of physical activity was described by 4 categories: (i) affected by study participation; (ii) impact of having heart failure; (iii) mixed feelings when physically active; and (iv) influence of the social and physical environment. Study participation made patients feel encouraged or obliged, or they gained a sense of security in connection with becoming more physically active. Symptoms, side-effects and barriers could hamper their physical activity. They changed their type and level of physical activity, or used resources or equipment to become physically active. Patients experienced physical and emotional improvements and developed routines and structures, and their environment and caregivers were able to influence their physical activity. Conclusion: The role of study participation, having heart failure, and psychological and environmental factors describe the experiences of physical activity in patients with heart failure who were randomized to a control group, and are important in evaluating and motivating patients’ physical activity.