The Rotterdam Elderly Study is designed as a community-based prospective follow-up study. The study population consists of almost 12,000 individuals aged 55 years and over who are living in an urban area in Rotterdam, the Netherlands. The objective of the study is to investigate the prevalence, incidence and determinants of neurogeriatric disorders, cardiovascular diseases, locomotor diseases and ophthalmologic diseases. In this paper we give a general outline of the Rotterdam Elderly Study, and a detailed description of the case finding and diagnostic procedures for the study of dementia. The risk factors for dementia that are assessed in the study are summarized.