Significant cognitive impairment affects approximately 15 percent of the U.S. population over age 65. The most prevalent form of irreversible dementia is senile dementia of the Alzheimer's type (SDAT), which accounts for at least 50 percent of nursing home admissions and roughly 10 billion dollars a year in health care costs. In spite of the social, economic, and medical impact of the disease, epidemiological data are scarce concerning risk factors associated with the disease. Until recently, most of the population-based studies of senile dementia in the elderly were carried out in Northern European countries. The methodological problems that arise in performing epidemiologic studies on SDAT may in part explain the lack of sufficient data pertaining to certain risk factors. This paper provides a review of the literature and research on risk factors in dementia in the elderly and discusses directions for future research. The epidemiological issues associated with certain key studies are also discussed. Although the major studies that have attempted to look at risk factors in mental illness in the elderly are beginning to fill in some of the gaps in the understanding of the etiology and epidemiology of the disease, further epidemiologic studies on both institutionalized and community-based elderly populations are needed. These should assess a wide range of possible factors (demographic, psychological, environmental, health practice, and medical) in terms of their association with SDAT. By developing a comprehensive epidemiologic profile of SDAT, possible preventive measures and therapeutic approaches to treating the disease may be identified and hypotheses for future epidemiological and laboratory investigations developed.