This study presents detailed estimates of the hours and cost of informal care provided to Alzheimer's disease patients in Northern California. Data were collected over 12 months from 93 community-residing patients and their caregivers and from 94 institutionalized patients and their caregivers. Patients residing in the community received an average of 286 hours per month of unpaid care, whereas institutionalized patients received 36 hours per month. Alternative methods of imputing informal costs are described. The annual value of informal care (in 1990 dollars) is $34,517 for the noninstitutionalized sample and $5,542 for the institutionalized sample. The determinants of informal caregiving were examined and compared in the two settings. The more cognitively impaired the individual, the higher the informal costs no matter what the residential setting. In an institutional setting, the younger the patient and caregiver, the higher the costs. If a caregiver is not a spouse, higher informal care costs were also found.