Objectives: This study aimed to estimate the costs of formal and informal care of patients with Alzheimer's disease, to compare care costs across European countries and identify potential differences in cost patterns between countries and regions. Setting: The ICTUS study is a prospective, naturalistic observational study conducted in specialised memory clinics in 12 European countries. In total, 1385 patients diagnosed with Alzheimer's disease were enrolled at baseline. All subjects had a reliable informant (primary caregiver) and informed consent was obtained from patients or their primary caregiver. Main outcome measures: Resource utilization data was captured with the RUD Lite (Resource Utilization in Dementia) instrument and caregiver burden with the Zarit Burden Interview (ZBI). Patient disease severity was measured with the Mini-Mental State Examination (MMSE), Alzheimer's Disease Assessment Scale-Cognitive subscale (ADAS-cog), Katz' index (PADL), Instrumental activities of daily living (IADL) scale and Neuropsychiatric inventory (NPI). Results: The mean annual cost of care per patient was estimated to €7,820 (95% CI: €7,194-€8,446), whereof 54% were costs of informal care, 16% direct medical costs and 30% community care costs. There were substantial differences in total resource utilization and also in the balance between formal and informal care between Northern, Western and Southern Europe. PADL scores were strongly associated with formal care costs while IADL scores correlated strongly with informal care costs. Conclusions: Costs of Alzheimer's Disease are high across European countries. Activities of daily living is an important determinant of care costs. Formal care service use is lower and informal care higher in Southern Europe compared to Western and Northern Europe. Differences in resource utilization patterns are important to consider in international studies of dementia care costs as well as in economic evaluations of new treatments for dementia.