Background: At the end of life, formal care costs are high. Informal care (IC) costs, and their effects on outcomes, are not known. This study aimed to determine the IC costs for older adults in the last 3 months of life, and their relationships with outcomes, adjusting for care quality. Methods: Mortality follow-back postal survey. Setting: Palliative care services in England (London), Ireland (Dublin) and the USA (New York, San Francisco). Participants: Informal carers (ICrs) of decedents who had received palliative care. Data: ICrs reported hours and activities, care quality, positive aspects and burdens of caregiving, and completed the Texas Revised Inventory of Grief (TRIG). Analysis: All costs (formal, informal) were calculated by multiplying reported hours of activities by country-specific costs for that activity. IC costs used country-specific shadow prices, e.g. average hourly wages and unit costs for nursing care. Multivariable logistic regression analysis explored the association of potential explanatory variables, including IC costs and care quality, on three outcomes: positive aspects and burdens of caregiving, and subsequent grief. Results: We received 767 completed surveys, 245 from London, 282 Dublin, 131 New York and 109 San Francisco. Most respondents were women (70%); average age was 60 years. On average, patients received 66-76 h per week from ICrs for 'being on call', 52-55 h for ICrs being with them, 19-21 h for personal care, 17-21 h for household tasks, 15-18 h for medical procedures and 7-10 h for appointments. Mean (SD) IC costs were as follows: USA $32,468 (28,578), England $36,170 (31,104) and Ireland $43,760 (36,930). IC costs accounted for 58% of total (formal plus informal) costs. Higher IC costs were associated with less grief and more positive perspectives of caregiving. Poor home care was associated with greater caregiver burden. Conclusions: Costs to informal carers are larger than those to formal care services for people in the last three months of life. If well supported ICrs can play a role in providing care, and this can be done without detriment to them, providing that they are helped. Improving community palliative care and informal carer support should be a focus for future investment.