The financial, opportunity and social costs and benefits of providing informal support to frail older people are described within an economic framework. Mentally and/or physically frail older people were identified through screening interviews with random community samples of people aged 65 and over in four UK areas: 884 frail older people living in private households nominated key informal supporters and 650 of these supporters were interviewed. Around half the supporters reported financial costs (43 per cent) or lost social opportunities (45 per cent). A minority of supporters had reduced their working hours or withdrawn from employment because of caregiving. Nearly all supporters reported at least one social cost (92 per cent) and identified at least one positive aspect of caregiving (95 per cent). Co-resident supporters were more likely to report opportunity costs and loss of health and well-being. Daughters supporting a frail parent were generally most likely, and friends or neighbours least likely, to report each type of cost. Supporters of older people who were both mentally and physically frail reported significantly greater opportunity and social costs. The benefits of caregiving were not consistently related to co-residency, relationship of supporter or frailty type. Possible strategies for decreasing the costs and increasing the benefits of caregiving are discussed.