Background: A lack of physical activity during hospitalization can lead to adverse outcomes like complications and loss of physical function. More insight into factors that influence physical activity during a hospital stay is needed to develop strategies to change the mobility culture in hospitals. Objective: To give an overview of factors that influence physical activity of patients by exploring the perspectives of both patients and health-care professionals regarding physical activity during hospital stay. Method: Semi-structured interviews with patients and health-care professionals were conducted at a university hospital in the Netherlands. Patients were interviewed about their daily activities during their hospital stay and the factors that were of influence. Health-care professionals were asked about their perceptions regarding their responsibilities in promoting physical activity during hospitalization. Results: In total eight patients and nine health-care professionals participated. Patients and health-care professionals stated that low physical activity levels were mostly caused by a poor physical status, patients' expectations to lie in bed during hospitalization, and the lack of knowledge on the importance of physical activity. Lack of time was the main barrier for health-care professionals to promote physical activity. Conclusion: Physical activity is not yet seen as a structural part of hospital care by both patients and health-care professionals, and does not have priority within current daily routines. To increase physical activity levels of hospitalized patients, more knowledge and tools should be available for both patients and health-care professionals.