Although delirium is a common complication in terminally ill cancer patients and can cause considerable distress for family members, little is known about effective care strategies for terminal delirium. The primary aims of this study were 1) to clarify the distress levels of bereaved families and their perceived necessity of care; and 2) to explore the association between these levels and family-reported professional care practice, family-reported patient behavior, and their interpretation of the causes of delirium. A multicenter questionnaire survey was conducted on 560 bereaved family members of cancer patients who developed delirium during their final two weeks in eight certified palliative care units across Japan. We obtained 402 effective responses (response rate, 72%) and, as 160 families denied delirium episodes, 242 responses were analyzed. The bereaved family members reported that they were very distressed (32%) and distressed (22%) about the experience of terminal delirium. On the other hand, 5.8% reported that considerable or much improvement was necessary, and 31% reported some improvement was necessary in the professional care they had received. More than half of the respondents had ambivalent wishes, guilt and self-blame, and worries about staying with the patient. One-fourth to one-third reported that they felt a burden concerning proxy judgments, burden to others, acceptance, and helplessness. High-level emotional distress and family-perceived necessity of improvement were associated with a younger family age; male gender; their experience of agitation and incoherent speech; their interpretation of the causes of delirium as pain/physical discomfort, medication effects, or mental weakness/death anxiety; and their perception that medical staff were not present with the family, not respecting the patient's subjective world, not explaining the expected course with daily changes, and not relieving family care burden. In terminal delirium, a considerable number of families experienced high levels of emotional distress and felt some need for improvement of the specialized palliative care service. Control of agitation symptoms with careful consideration of ambivalent family wishes, providing information about the pathology of delirium, being present with the family, respecting the patient's subjective world, explaining the expected course with daily changes, and relieving family care burden can be useful care strategies.