In this paper the Rotterdam Elderly Study is presented. The aim of the study is to investigate determinants of disease occurrence and progression in the elderly. In addition to contributing to our understanding of the etiology of geriatric illnesses, the study is expected to lead to specific recommendations for intervention. The study focuses on causally related determinants of major diseases in the elderly. Fields of interest for the Rotterdam Elderly Study are conditions which interfere the most with the quality of life for the elderly. The aims of the Rotterdam Elderly Study are: (1) To investigate, by means of epidemiologic, clinical and basic research, the determinants of diseases in order to assess their etiologic significance. (2) To investigate potentially modifiable determinants in order to be able to develop preventive strategies by providing specific recommendations for intervention studies. The Rotterdam Elderly Study focuses on four primary areas of research: neurogeriatric diseases, cardiovascular diseases, locomotor diseases and ophthalmologic diseases. It is a prospective follow-up study, in which determinants of disease and determinants of progression of disease will be investigated in the total population of 55 years or over of the district of Ommoord in Rotterdam. It is anticipated that about 10,000 people will participate in the study and they will be examined in the period of 1991 to 1995.