Aims: The aims of this study are to describe informal care activities and to estimate the societal cost of informal care of community-dwelling frail elderly people in Sweden. Methods: This study was performed within the frame of the TREEE project that included 408 frail elderly patients. At index hospitalisation (baseline), primary informal caregivers of the patients were provided with a questionnaire on informal care during a period of three months. Questions concerning other (secondary) informal caregivers were also included. A rough estimate of the total cost of informal care of frail elderly people in Sweden was obtained by combining data from this study with published data and official statistics. Results: In total, 176 informal caregivers responded, and 89% had provided informal care. The informal caregivers (primary and secondary) provided care for an average of 245 hours over three months. Taking care of the home was the dominating activity. In total, the mean cost of informal care was estimated to approximately 18,000 SEK (euro1878) over three months, corresponding to an annual cost of approximately 72,000 SEK (euro7477) per frail elderly person. The total annual societal costs of informal care of community dwelling frail elderly people aged 75 years and older in Sweden was estimated to be approximately 11,000 million SEK (euro1150 million). Conclusions: The care of frail elderly people provided by informal caregivers is extensive and represents a great economic value. Although our calculations are associated with uncertainty, the size indicates that supporting informal caregivers should be a priority for society.