Purpose: We document the extent to which older adults accurately report their spouses' end-of-life treatment preferences, in the hypothetical scenarios of terminal illness with severe physical pain and terminal illness with severe cognitive impairment. We investigate the extent to which accurate reports, inaccurate reports (i.e., errors of undertreatment or overtreatment), and uncertain reports (responses of "do not know") are associated with spouses' advance care planning and surrogates' involvement in the planning. Design and Methods: We used data from married couples who participated in the Wisconsin Longitudinal Study in 2004. These 2,750 couples were in their mid-60s and in relatively good health. We conducted multinomial logistic regressions. Results: Surrogates were accurate in the majority of cases, made errors in 12% to 22% of cases, and were uncertain in 11% to 16% of cases. Errors of overtreatment and undertreatment were equally prevalent. For both scenarios, discussing preferences was associated with lower odds of an uncertain surrogate response. Implications: We suggest ways that health care practitioners could facilitate family-level conversations in order to ensure that patients' preferences are accurately represented in end-of-life care settings.