Research has shown that involving patients in decisions on treatment may have positive effects for patients. However, there are different understandings of what involving patients implies and different attitudes among physicians toward sharing decisions with patients. This study aimed to explore the attitude of patients with advanced prostate cancer to involvement in treatment decisions and physicians’ and nurses’ approach to patient involvement. Moreover, it aimed to explore whether the decision-making changes as the treatment course progresses. Data were collected through participant observations and interviews. It was found that treatment decisions are primarily considered as part of physicians’ role. Physicians’ attitudes to patient involvement in treatment decisions depended on the type of treatment. Among patients and health professionals, there was a lack of confidence in the ability of patients to participate in the decisions. Health professionals and patients have different attitudes toward patient involvement and different views on what it entails.