OBJECTIVE: There is no tool to appropriately assess terminal delirium, including the natural terminal course. The objective of this study was to develop an evaluation scale to assess distress from irreversible terminal delirium and to examine the validity of the scale. METHOD: Based on previous qualitative analysis and systematic literature searches, we carried out a survey regarding the views of bereaved families and developed a questionnaire. We extracted items that bereaved families regarded as important and constructed an evaluation scale of terminal delirium. Then, we applied the questionnaire in a cross-sectional questionnaire survey of bereaved relatives of cancer patients who were admitted to a hospice or a palliative care unit. RESULTS: We developed the Terminal Delirium-Related Distress Scale (TDDS), a 24 item questionnaire consisting of five subscales (support for families and respect for a patient, ability to communicate, hallucinations and delusions, adequate information about the treatment of delirium, and agitation and restlessness). Two hundred and eighty-one bereaved relatives participated in the validation phase. The construct validity was shown to be good by repeated factor analysis. Convergent validity, confirmed by the correlation between the TDDS and the Care Evaluation Scale (r = 0.651, P < 0.001), was also good. The TDDS had good internal consistency (Cronbach's alpha coefficient for all 24 items = 0.84). SIGNIFICANCE OF RESULTS: This study showed that the TDDS is a valid and feasible measure of irreversible terminal delirium.