Abstract Background Relatives have no formal position in the practice of euthanasia and physician-assisted suicide (EAS) according to Dutch legislation. However, research shows that physicians often involve relatives in EAS decision-making. It remains unclear why physicians do (not) want to involve relatives. Therefore, we examined how many physicians in the Netherlands involve relatives in EAS decision-making and explored reasons for (not) involving relatives and what involvement entails. Methods In a mixed-methods study, 746 physicians (33% response rate) completed a questionnaire, and 20 were interviewed. The questionnaire included two statements on relatives’ involvement in EAS decision-making. Descriptive statistics were used, and multivariable logistic regression analyses to explore characteristics associated with involving relatives. In subsequent interviews, we explored physicians’ views on involving relatives in EAS decision-making. Interviews were thematically analysed. Results The majority of physicians want to know relatives’ opinions about an EAS request (80%); a smaller group also takes these opinions into account in EAS decision-making (35%). Physicians who had ever received an explicit EAS request were more likely to want to know opinions and clinical specialists and elderly care physicians were more likely to take these opinions into account. In interviews, physicians mentioned several reasons for involving relatives: e.g. to give relatives space and help them in their acceptance, to tailor support, to be able to perform EAS in harmony, and to mediate in case of conflicting views. Furthermore, physicians explained that relatives’ opinions can influence the decision-making process but cannot be a decisive factor. If relatives oppose the EAS request, physicians find the process more difficult and try to mediate between patients and relatives by investigating relatives’ objections and providing appropriate information. Reasons for not taking relatives’ opinions into account include not wanting to ...