This study was designed to identify beliefs about physical activity in cancer patients. Semi- structured interviews were conducted with 20 patients under treatment, who were invited to identify perceived barriers for not adopting a physically active lifestyle and to express their beliefs about physical activity. Content analyses revealed five main categories of beliefs including four types of barriers: (a) barriers related to the side effects of treatment; (b) barriers related to a lack of perceived physical abilities; (c) barriers related to a lack of interest for physical activity; (d) beliefs about the negative effects of physical activity, and (e) beliefs about the positive effects of physical activity. These findings extend the existing literature by indicating how stereotypes may play a role in explaining sedentary lifestyles in cancer patients.