Following a diagnosis of cancer, patients are often faced with important medical decisions regarding their treatment. This decision-making process can be difficult as patients may not receive sufficient information about their options or may struggle to interpret the information they do receive. An unsatisfactory decision may lead to later decisional regret, which has been linked to a lower quality of life in survivors. While some research has begun to explore the factors that influence decisional regret, few studies have comprehensively examined the role that psychological and social factors may play in this process. The aim of the current project was to analyse and compare the factors associated with the emergence of decisional regret in cancer survivors internationally and in Ireland. Two studies were conducted. Study 1, a systematic review, analysed international literature aimed at exploring factors associated with decisional regret in 27,982 cancer patients and survivors. Study 2, a survey, explored factors associated with regret in 92 cancer survivors in Ireland (the majority who were breast and prostate cancer survivors). In this study, sociodemographic, health, psychological and social support information was analysed in its ability to predict regret as measured by the Decision Regret Scale. Public and patient involvement was used to aid the interpretation of the findings. In Study 1, regret in cancer patients/survivors was associated with various sociodemographic factors, poorer physical health, a number of treatment types, an unsatisfactory decision-making process, poorer mental health and lack of social support. More generally, these factors were grouped into those which were modifiable and less modifiable from the patient’s perspective. These results were mirrored in Study 2, with better physical and psychological well-being, higher levels of shared-decision making and resilience being the 3 factors which most strongly predicted regret